When Stephen Miller met with Mark Zuckerberg at Mar-a-Lago late last year, the 39-year-old Trump adviser was in a position of power that would have been unimaginable a decade ago.

Back then, Mr. Miller was a mere Senate staffer railing about the evils of immigration. Now he was holding forth on U.S. policy with the billionaire chief executive of Meta, a man he had vilified for years as a globalist bent on destroying the nation.

The scale had flipped.

Mr. Miller told Mr. Zuckerberg that he had an opportunity to help reform America, but it would be on President-elect Donald J. Trump’s terms. He made clear that Mr. Trump would crack down on immigration and go to war against the diversity, equity and inclusion, or D.E.I., culture that had been embraced by Meta and much of corporate America in recent years.

Mr. Zuckerberg was amenable. He signaled to Mr. Miller and his colleagues, including other senior Trump advisers, that he would do nothing to obstruct the Trump agenda, according to three people with knowledge of the meeting, who asked for anonymity to discuss a private conversation. Mr. Zuckerberg said he would instead focus solely on building tech products.

Mr. Zuckerberg blamed his former chief operating officer, Sheryl Sandberg, for an inclusivity initiative at Facebook that encouraged employees’ self-expression in the workplace, according to one of the people with knowledge of the meeting. He said new guidelines and a series of layoffs amounted to a reset and that more changes were coming.

Earlier this month, Mr. Zuckerberg’s political lieutenants previewed the changes to Mr. Miller in a private briefing. And on Jan. 10, Mr. Zuckerberg made them official: Meta would abolish its D.E.I. policy.

The meeting at Mar-a-Lago on Nov. 27 represented more than just another tech billionaire bending the knee to Mr. Trump. It vividly demonstrated the power and influence of Mr. Miller, who in less than a decade has risen from an anti-immigrant agitator on Capitol Hill to one of the most powerful unelected people in America.

Officials from Meta declined to comment, as did Mr. Miller. A Trump transition spokeswoman declined to address a majority of the reporting.

Mr. Miller was influential in Mr. Trump’s first term but stands to be exponentially more so this time. He holds the positions of deputy chief of staff, with oversight of domestic policy, and homeland security adviser, which gives him range to coordinate among cabinet agencies. He will be a key legislative strategist and is expected to play an important role in crafting Mr. Trump’s speeches, as he has done since he joined the first Trump campaign in 2016.

Most significantly, Mr. Miller will be in charge of Mr. Trump’s signature issue and the one that Mr. Miller has been fixated on since childhood: immigration. And he has been working, in secrecy, to oversee the team drafting the dozens of executive orders that Mr. Trump will sign after he takes office on Jan. 20.

“I call Stephen ‘Trump’s brain,’” said Kevin McCarthy, the former House speaker who credited Mr. Miller — a private citizen at the time — with helping to rally Republican lawmakers to insert a sweeping border crackdown into a spending bill in 2023.

In the four years since Mr. Trump has been out of office, Mr. Miller has spent more time than any close Trump adviser mapping out a second-term playbook. He expanded on the hard-line first-term immigration policies; he deepened his relationships with House members, senators and influential right-wing media figures; he built a nationwide donor network to fund a nonprofit that he used as an additional tool of influence; and he quietly cultivated a relationship with the richest man in the world, Elon Musk.

Mr. Miller will re-enter government with even more trust and credibility with the president, fewer internal rivals and a more expansive team reporting to him.

Those who dealt with — and often dismissed — Mr. Miller a decade ago when he was a young Senate staffer, emailing reporters late at night on behalf of Senator Jeff Sessions of Alabama, with lurid stories about immigrants committing crimes, can hardly believe the scope of his power.

Taking Charge

After Mr. Trump won the election in November, Mr. Miller moved his family down to Palm Beach, Fla., and took a major role in the transition.

People briefed on the executive orders that his team is drafting say they include an attempt to end birthright citizenship; a designation of drug cartels as foreign terrorist organizations; and a reinstatement of Title 42, which allows the United States to seal the border with Mexico if there is a public health threat. (Mr. Trump’s advisers have spent months trying to identify a disease that will help them build a case for Title 42, since there is no such emergency at the moment.)

It will be up to Mr. Trump to decide which orders to issue, but Mr. Miller is focused on immigration. The homeland security adviser’s other responsibilities include dealing with natural disasters like the one raging in California, his home state. (The fires destroyed Mr. Miller’s parents’ home, people close to him said.) Mr. Miller is expected to shift some of his portfolio to the national security adviser.

As he works out his priorities, Mr. Miller appears to have learned two key lessons from the first Trump term.

The first is to flood the zone. He believes that those he regards as Mr. Trump’s enemies — Democrats, the media, groups like the American Civil Liberties Union and portions of the federal bureaucracy — are depleted and only have so much bandwidth for outrage and opposition. Mr. Miller has told people that the goal is to overwhelm them with a blitz of activity.

The second lesson has been to operate with as much secrecy as possible to prevent anyone from finding ways to obstruct the Trump agenda. As a congressional staffer, Mr. Miller was freewheeling in his digital communications. But since working for Mr. Trump, who doesn’t use email and regards people who take notes with suspicion, he puts almost nothing in writing. Instead, he works through emissaries.

The protectiveness around the executive orders is particularly notable. An incoming administration would usually send the drafts to the Justice Department’s Office of Legal Counsel, where a career lawyer — walled off from the outgoing administration’s political appointees — reviews them for form and legality and suggests improvements. For the most part, Mr. Trump’s first transition is said to have followed that practice.

But Mr. Miller is using a team of lawyers from outside the Justice Department to vet the orders, a person with knowledge of the situation said — a sign of Trump aides’ general distrust of the Justice Department, which brought three special counsel investigations into Mr. Trump and twice indicted him.

In the meantime, Mr. Miller is trying to eliminate any roadblocks to Mr. Trump’s immigration plans. Mass deportations will require arrangements with other countries to take in the migrants; to that end, Mr. Miller lobbied for his ally, the former ambassador to Mexico, Christopher Landau, to be chosen as deputy secretary of state under Marco Rubio, the Florida senator whom Mr. Trump has chosen to lead the agency.

Knowing the White House will need billions in congressional appropriations for the biggest deportation operation in American history — which he’s previously said will include sweeping raids and use of the U.S. military to build massive camps to detain the migrants — Mr. Miller has spent the past four years building relationships with lawmakers.

It appears to have paid off.

When Mike Johnson addressed the House Republican conference after securing the speakership, he made a point of singling out Mr. Miller for praise. Senator Jim Banks of Indiana, a former House member, said he talked to Mr. Miller nearly every day for the four years that Mr. Trump was out of the White House. And Senator Mike Lee of Utah said there had been many times he pondered a new policy, when “all of a sudden a thought will occur to me: I wonder what Stephen Miller thinks of this one.”

The Long Game

The last time Mr. Miller participated in a Trump transition, after the surprise victory of 2016, he was fairly low in the Washington power structure.

He had become a minor celebrity on the right in 2006 for vocally defending a group of Duke University lacrosse players who had been accused — falsely, it later became clear — of rape. But he was best known to insiders as the scrappy congressional staffer for Mr. Sessions. Much of Washington’s establishment regarded Mr. Miller as a racist, and as an irritant, mocking his over-the-top pronouncements and skinny ties.

He joined the Trump campaign part time in late 2015 and full time in early 2016, one of a handful of original aides on a small team. He worked like a man possessed, staying up all night to write Mr. Trump’s speeches, a task assigned to him by Mr. Trump’s son-in-law Jared Kushner. He channeled Mr. Trump’s voice better than any other adviser.

But he entered the executive branch knowing little about how it worked, and it showed. The travel ban executive order against mostly Muslim-majority countries, crafted in secret by an ally of Mr. Miller’s amid concern some Trump appointees would try to stop it, was criticized as sloppily drafted and was initially blocked by the courts.

Mr. Miller mostly stayed out of the factional warfare that defined the early years of Mr. Trump’s first term. He was friendly with the more moderate West Wing camp — people like Mr. Kushner and Hope Hicks — and with those on the sharp edge of Mr. Trump’s movement.

People who have worked closely with Mr. Miller say they cannot recall him ever expending his political capital on an ally who fell out of favor with Mr. Trump. When Mr. Sessions, his former boss who was now attorney general, became persona non grata with Mr. Trump over the Russia investigations, Mr. Miller made it clear that his allegiance was to the president.

His strategy paid off. He survived. And his vision for immigration — including deeply restrictive and xenophobic policies — are now at the center of Mr. Trump’s economic and cultural agenda.

Unlike many others, he stuck with Mr. Trump after the violence of Jan. 6, 2021, when a mob of Trump supporters stormed the Capitol. He remained a paid adviser and a frequent Fox News presence promoting the Trump agenda, and made an early public endorsement of Mr. Trump’s 2024 presidential campaign at a time when many Republicans wanted to move on.

Mr. Miller, who comes from a wealthy family, did something else that Mr. Trump appreciated: He did not try to leverage his Trump ties into lucrative consulting contracts. The compensation he drew from his nonprofit, the America First Legal Foundation, in 2023 — $266,000 — was far less than what he could have earned working as a political gun for hire.

“Some people in Trump’s world have been there for career advantage or transactional reasons,” said Charlie Kirk, the conservative activist who is close to both Mr. Trump and Mr. Miller. “But Stephen believes in the president’s agenda deeply.”

He plays the long game on relationships, scouting people who may be influential several years in the future. He built a relationship with JD Vance ahead of his successful Ohio Senate primary, years before he would become Mr. Trump’s running mate.

He also can be a political shape-shifter when it’s expedient for him.

His long-term demonization of “radical Islam” went relatively quiet at moments during the 2024 presidential race, as he encouraged the Trump campaign to issue inviting statements to Muslims in Michigan — part of a strategy to exploit Muslims’ anger over the Biden administration’s support for Israel, according to three people with direct knowledge.

Mr. Miller is generally well-liked on the Trump staff, though he is regarded as unusually intense and has been known to berate government officials he deemed obstructive. He has strongly held opinions about even minor matters, like men’s fashion. Specifically: fabrics, patterns, colors and collars.

He never argues with Mr. Trump, certainly never in front of others. Once it’s clear to him that Mr. Trump is headed in a certain direction, he sets aside his reservations.

In recent weeks, according to multiple people with direct knowledge, Mr. Miller has done little, if anything, to try to talk Mr. Trump out of his support for H-1B visas to import high-skilled foreign workers — despite the fact that Mr. Miller has spent much of his career condemning such visas.

Another recent example: Mr. Miller was initially surprised that Kristi Noem, the South Dakota governor, was chosen by Mr. Trump for secretary of the Department of Homeland Security. Mr. Miller had wanted Thomas D. Homan, whom Mr. Trump had picked as his border czar, for the D.H.S. role, according to two people who spoke to him at the time. But when it was clear Mr. Trump was set on the idea, he did not try to dissuade him.

“He has the president’s complete trust,” said Mr. McCarthy. “Trump’s complained about everyone. Never him.”

Mr. Trump may not complain about Mr. Miller, but he does occasionally poke at his obsession with immigrants — a hostility that goes far beyond Mr. Trump’s. In one meeting during the 2024 campaign, Mr. Trump said that if it was up to Mr. Miller there would be only 100 million people in this country, and they would all look like Mr. Miller, according to a person with knowledge of the comment. Karoline Leavitt, Mr. Trump’s spokeswoman, denied the account.

The Outside-In Strategy

Since he was a high schooler in Santa Monica, Calif., obsessed with Rush Limbaugh, Mr. Miller has cultivated right-wing media personalities. He is close to Tucker Carlson and Fox News’s Laura Ingraham, but he also follows the new wave of podcasters and comedians.

Mr. Miller has told friends how pleased he is that the Trump movement has shifted the cultural dial on his favored policies. Prominent Democrats have scrambled to rebrand themselves as tough on immigration, and officials such as New York City’s mayor, Eric Adams, have welcomed tighter restrictions after an influx of migrants in their cities.

Mr. Miller has spent much of the past four years figuring out how to build pressure from outside of government to help enact Mr. Trump’s agenda.

Less than a month after Mr. Trump left office, he founded the America First Legal Foundation, a nonprofit “public interest law firm.” Mr. Miller, who is not a lawyer himself, cast the group as a conservative answer to the American Civil Liberties Union, helping the little guy fight big government or big tech.

His group quickly became a fund-raising powerhouse, raising $44 million in 2022.

Mr. Miller’s group used some of that money on legal work. It filed more than 100 lawsuits, legal briefs and other actions, and helped block a Biden administration plan to offer debt relief to Black farmers, which Mr. Miller’s group said was discriminatory.

But it spent far more on advertising: $32 million, which was nearly 70 percent of its total spending. Some of those ads seemed designed to damage Democrats in the run-up to elections. In 2022, for instance, the group paid for ads in swing states that accused the Biden administration of “anti-white bigotry.”

Now, as Mr. Trump returns to the White House, the America First Legal Foundation wants to serve as an attack dog for the Trump administration. In December, the group sent letters to 249 city and state officials in “sanctuary” jurisdictions that have said they will not cooperate with federal immigration authorities to help them arrest immigrants. If these officials do not participate in Mr. Trump’s crackdown, Mr. Miller’s group said, the local officials could be considered to be illegally “harboring” undocumented immigrants.

Experts said it would be difficult for the group to actually sue local officials, but, as before, Mr. Miller’s group is contemplating a campaign outside the courtroom. It filed public-records requests with 17 states and cities, seeking evidence that they were preparing to defy Mr. Trump’s crackdown. And it set up a website called “Sanctuary Strongholds,” designed to direct public pressure against state and local officials.

Key to some of those outside efforts will be one of the relationships Mr. Miller has established in the last few years — an alliance almost as valuable as his one with Mr. Trump. Mr. Miller found common cause with Mr. Musk, who had begun describing undocumented immigrants as a threat to Western civilization. Mr. Miller’s wife, Katie, is also working with Mr. Musk, at his so-called Department of Government Efficiency.

Mr. Miller began advising Mr. Musk on his political donations, which were at the time a closely held secret, according to two people with knowledge of the matter. A nonprofit called Citizens for Sanity, which tax filings show is closely tied to Mr. Miller’s group, raised $94 million in 2022 and paid for ads that attacked Democrats’ policies on transgender youth. The Wall Street Journal reported that $50 million of the donations to Citizens for Sanity that year came from an outside group that Mr. Musk had been donating to. The America First Legal Foundation and Citizens for Sanity did not respond to questions sent by The New York Times.

Mr. Miller is also secretive about his relationship with Mr. Musk. But one person willing to discuss it on condition of anonymity said Mr. Musk had once told him: “I want doers. And most of these people in government, that’s not how they are.”

The person recalled that Mr. Musk allowed for one exception: “But Stephen Miller — I love Stephen Miller. He’s a doer.”

Annie Karni contributed reporting from Washington.