In December 2023, Angela McArdle, the chair of the Libertarian Party, flew to Mar-a-Lago to meet with Donald J. Trump.

Mr. Trump wanted to know how to win over libertarian voters, a constituency he thought could help him reclaim the presidency, Ms. McArdle said in an interview. She had an answer: Free Ross Ulbricht, a Bitcoin pioneer who was sentenced to life in prison in 2015 for creating Silk Road, the world’s largest online drug marketplace. Mr. Ulbricht was regarded as a libertarian hero for building an illegal market outside the government’s reach.

“I love freeing people,” Mr. Trump said, according to Ms. McArdle. Five months later, she hosted him at the Libertarian Party’s national convention, where he announced onstage that, if elected to the presidency, he would release Mr. Ulbricht.

On Tuesday, the day after his inauguration, Mr. Trump made good on that promise. He called Mr. Ulbricht’s mother, Lyn Ulbricht, to personally tell her that he had granted a full pardon to her son, who is now 40. In a post on Truth Social, Mr. Trump said the decision was “in honor of her and the Libertarian Movement, which supported me so strongly.”

Mr. Ulbricht’s pardon was not an obvious agenda item for Mr. Trump. Unlike the nearly 1,600 people who received pardons or commutations this week for their involvement in the Jan. 6 riot, Mr. Ulbricht had little direct connection to the president. But the move had long been in the works, after more than a decade of activism by Mr. Ulbricht’s supporters — including cryptocurrency investors, libertarian politicians and especially Ms. Ulbricht, who was a vocal proponent for her son’s release.

Many of them have enjoyed an unusual level of access to Mr. Trump. As it became clear last year that Mr. Trump would be the Republican nominee, they waged a behind-the-scenes lobbying campaign to secure a pardon — including pledging to raise money for his election bid — in what has turned into a case study of how a special interest group can mobilize to influence the president.

Ms. McArdle said she was put in contact with Mr. Trump by Richard Grenell, one of his longtime advisers and a former acting director of national intelligence, who suggested she treat conversations with Mr. Trump like a business negotiation.

“Ric was like, ‘He’s a deal-maker, Angela,’” she said. “Don’t be afraid to ask for something.”

Mr. Grenell, Ms. Ulbricht and the Trump administration did not respond to requests for comment.

Mr. Ulbricht’s pardon shows “that if you have a concentrated base of people around Trump, you have a very good chance at a pardon,” said Dan Richman, a former federal prosecutor who teaches at Columbia Law School. “There are problems with the pardon system working that way.”

Mr. Ulbricht launched Silk Road in 2011 and turned it into one of the most popular outposts of the so-called Dark Web, a hidden corner of the internet that people can access only through a special browser. Silk Road facilitated over 1.5 million transactions, generating more than $200 million in revenue from the sale of heroin, methamphetamine, cocaine and other drugs, authorities have said. Users transacted anonymously with Bitcoin, then a nascent cryptocurrency, and could post Amazon-style product ratings.

In 2013, the F.B.I. arrested Mr. Ulbricht at a San Francisco library and charged him with running Silk Road. In court, prosecutors presented evidence that Mr. Ulbricht had also solicited the murders of people he considered threats to the business, though he was never tried on murder-for-hire charges and there was no indication that any killings took place.

At least six deaths were attributed to drugs bought on Silk Road, prosecutors said in court. A federal judge in the Southern District of New York, where the case was tried, called Mr. Ulbricht “the kingpin of a worldwide digital drug-trafficking enterprise” whose actions were “terribly destructive to our social fabric.” In 2015, he received a life sentence for drug distribution, money laundering and other charges, and was eventually moved to a federal prison in Arizona.

The punishment struck some legal experts as harsh. It also drew protests from libertarians who opposed severe drug penalties and crypto enthusiasts who viewed Mr. Ulbricht as a pioneer.

Silk Road “onboarded a million people to Bitcoin,” said David Bailey, the chief executive of the news publication Bitcoin Magazine, who campaigned for Mr. Ulbricht’s release. “He represents many of the ideological views of our community.”

From prison, Mr. Ulbricht played up his connection to Bitcoin. In October 2018, he sent a letter to his mother celebrating the 10th anniversary of the cryptocurrency’s founding and likened himself to a “proud parent” of the technology.

“I guess I’m the estranged father in prison though, who can’t be there to help raise his kid,” he wrote in the letter, which was later published by Bitcoin Magazine.

On social media accounts maintained by his family, Mr. Ulbricht also shared artwork, updates on his prison gardening and thoughts on new technologies. The accounts posted links to online petitions asking for clemency, tagging Mr. Trump and Trump family members.

Behind the scenes, Ms. Ulbricht worked to popularize the slogan “Free Ross,” which become a rallying cry at crypto conferences. She also networked with Republican politicians and far-right influencers, hoping to reach Mr. Trump’s inner circle.

After he lost the 2020 election, Mr. Trump considered freeing Mr. Ulbricht, and at least one lobbyist was paid $22,500 to help secure his release, according to financial forms. But Mr. Trump left office without taking action.

“The higher the hope, the greater the disappointment, and our hopes were sky high for a commutation of sentence,” Mr. Ulbricht’s family posted on social media in January 2021.

The new Republican presidential campaign offered a fresh opportunity.

In 2023, Ms. Ulbricht renewed her push to connect with influential Republicans, including Vivek Ramaswamy, who was running for president, two people close to her said. Mr. Ramaswamy, who did not respond to a request for comment, committed to freeing Mr. Ulbricht if elected and spoke openly about meeting his mother.

Then in late 2023, Ms. McArdle was contacted by Mr. Grenell, who asked on behalf of Mr. Trump for advice on courting the libertarian vote, she said. Soon she was on a plane to Florida to meet Mr. Trump.

At the meeting, Ms. McArdle told Mr. Trump that Mr. Ulbricht was the victim of prosecutorial overreach and a biased criminal justice system, echoing complaints that the former president had made since leaving office.

“It’s the same court stuff in New York that has been giving you a hard time,” she said she told him.

Last year, Mr. Trump and his staff also met with Mr. Bailey and other representatives of Bitcoin Magazine, who pushed for Mr. Ulbricht’s release. Tracy Hoyos-López, who worked for the magazine, has said publicly that the introduction was arranged by Paul Manafort, Mr. Trump’s campaign chairman in 2016. (Ms. Hoyos-López is the daughter of Hector Hoyos, a friend and former business partner of Mr. Manafort.)

On social media, Mr. Bailey announced that he planned to raise a “$100m war chest for the Trump campaign.” He also went to Mar-a-Lago in June, he said in an interview, where he presented Mr. Trump with a letter from Lyn Ulbricht.

By then, Mr. Trump had already vowed to free Mr. Ulbricht at the Libertarian Party convention. He doubled down on that pledge in July at a conference in Nashville organized by Bitcoin Magazine, saying he would commute Mr. Ulbricht’s sentence — allowing him to walk free, but without erasing the conviction. Around that time, Mr. Trump also met privately with Ms. Ulbricht, said Ms. McArdle, who was briefed on the meeting.

Ms. McArdle has faced blowback from other libertarians for her dealings with Mr. Trump. But she was still in touch with the new administration last week, and requested that Mr. Trump grant Mr. Ulbricht a full pardon, not just a commutation. “Promises made, promises kept,” a Trump staffer emailed her, according to a copy of the message viewed by The New York Times.

On Tuesday night, Ms. McArdle, Mr. Bailey and Ms. Hoyos-López gathered in a livestream on X to wait for updates. Mr. Bailey told listeners that Ms. Ulbricht was in Arizona, preparing for her son’s release.

Within hours of the pardon, an account on X controlled by Mr. Ulbricht’s family posted a photograph of him leaving prison with a small plant and a sack of belongings.

“FREEDOM!!!!” the post said.

Kenneth P. Vogel contributed reporting. Susan C. Beachy contributed research.