Mr. Lee, an unlikely hero in a development battle that remains a storied chapter in Boston history, died Monday night in Beverly Hospital of cardiac arrest. He was 99 and had divided his time between Beacon Hill and Beverly Farms.

For more than 40 years, he led the Friends of the Public Garden from its origins in a Back Bay living room to its pivotal role as a powerful guardian of Boston landmarks.

Having served in the US Foreign Service earlier in his life, Mr. Lee brought his diplomatic training to bear on negotiating with city and state officials to form private-public partnerships that protected the Public Garden and other green spaces.

And though he worked tirelessly to oppose developer Mort Zuckerman’s initial Park Plaza proposal, Mr. Lee was just as persistent at deflecting acclaim for his own efforts.

“It’s not just modesty,” he told the Globe in 1995, adding that he believed it was “counterproductive” for him to be the focus of too much praise. “Many people work very hard for the Friends,” he said. “I do all the talking, and I get the credit.”

Mr. Lee did more than simply talk, however. During his presidency, he refused to take a salary and ran much of the operation from his home — even typing all correspondence as the organization expanded its activities beyond protection and upkeep of the Public Garden.

“He used to say, ‘We can either spend money on the parks or we can have an office,’ ” said Liz Vizza, his successor as president.

As president, Mr. Lee sometimes personally injected a treatment into trees along the Commonwealth Avenue Mall to blunt the blight of Dutch elm disease. As president emeritus in his 90s, he’d stop by the organization’s office to let everyone know a park bench needed spiffing up or a plaque had slipped from its holder.

“Henry’s combination of sweeping vision with incredible persistence and hard work to get all the little details right has transformed such an important part of how we think of Boston and how we interact with one of the most foundational spaces in the city,” said Mayor Michelle Wu.

Wu praised “the care and attention” he brought “to making sure everyone all across the city can experience beauty in their day-to-day lives and find that pop of color or calming presence. He devoted his life to making sure all of us would have that gift.”

Though born into a household of very modest means, Mr. Lee carried himself with a Boston Brahmin bearing, and his extended family was woven into the city’s history. Then-Governor Frank Sargent, who supported the original Park Plaza proposal, was a relative.

After graduating from Harvard College as a scholarship student and finishing graduate work, Mr. Lee spent several years in the US Foreign Service, which provided essential diplomatic training for his future civic work.

“Henry was a mentor and an inspiration to me and to so many of us in the parks advocacy world,” Vizza said. “He really taught me the true meaning of diplomatic advocacy.”

In an increasingly acrimonious world, Mr. Lee wrapped in a blanket of polite charm his steely will to preserve history and places where anyone and everyone could gather.

“He would often say, ‘You can beat down the doors of City Hall once, but the next time you go they’re going to change the locks,’ ” Vizza recalled. “He knew that to succeed you had to work with people.”

Born in Boston on Jan. 13, 1925, Mr. Lee was the son of Henry Lee and Frances LeMoyne Lee. Though Mr. Lee, his oldest son, and his father shared a first name, none of them used Jr. or III to highlight which generation.

Mr. Lee’s father had been a stellar runner and equestrian steeplechase competitor, but injuries from riding kept him from holding most jobs. His mother had raised two children from a previous marriage in addition to Henry.

When Mr. Lee was a boy, serious ear infections left him so weak that doctors thought he wouldn’t live past 12. “I think he sort of put that forecast to shame,” said his son Henry of Brookline.

Mr. Lee attended the Brooks School in North Andover, took a year off to work in Arizona earning money for Harvard, and then graduated with a bachelor’s degree. He also received a master’s in history from Stanford University.

In 1945, while a Harvard sophomore, Mr. Lee married Joan Cabot Metcalf, whom he had met at a party.

“Dad said, ‘If I didn’t marry her, I would have lost her,’ ” their daughter Karen of Portland, Ore., told The Beacon Hill Times earlier this year.

Joan Lee, a philanthropist who held behind-the-scenes roles in civic organizations, died in 2019.

Mr. Lee’s Foreign Service postings took him to Germany and Washington, D.C., before the family moved back to Boston in the late 1950s.

He taught at Browne and Nichols School in Cambridge and was a teacher and administrator at the Dexter School in Brookline, which overlapped into the civic activism that made him a public figure as he turned to it full-time in retirement.

The Friends of the Public Garden currently has more than 3,000 members from 32 states and an annual budget of $3.6 million. Since 1970, the group has helped raise and invest about $50 million in funds for specific allocations, including maintaining trees and the upkeep of sculptures and sustaining other green spaces.

This year, the organization established the Henry Lee Fund for Boston Parks to honor his legacy of commitment.

Along with the Friends, Mr. Lee led organizations such as the Massachusetts Historical Society and the Charitable Irish Society and a host of other groups.

“I used to kid him: ‘Have you ever been in an organization you weren’t head of,’ ” said his son, a senior lecturer at the Harvard Kennedy School.

Mr. Lee, his son said, “was my guiding light. He was the one who taught me where the bar is in life: ‘If you want to be a success, this is what you have to do to achieve it.’ “

A highly successful swimmer as a youth, Mr. Lee won numerous victory trophies, but into adulthood he kept only one — for a second-place finish.

“He was in lane seven and thought he was so far ahead of everybody else, but he forgot there was a guy in lane eight, who won,” his son said. “For his whole life he kept that trophy to remind himself that you cannot do anything without giving 100 percent.”

A service will be announced for Mr. Lee, who in addition to his son Henry and daughter Karen leaves another daughter, Eliza Schierloh of Beaumont, Texas; another son, Thomas of Seattle; 10 grandchildren; and six great-grandchildren.

“To a real extent his career began at 54,” Mr. Lee’s son Henry said.

“When Park Plaza fell into his lap,” he added, “all of a sudden all these civic opportunities came his way and he made the best of them. When you walk through the Public Garden today, you look around and say, ‘A part of this is because of him.’ “

Despite all of accomplishments, Mr. Lee was unwilling to boast even a little.

“Although I have never amounted to much, I have had interesting careers in government, education, and numerous civic and historical endeavors,” he wrote in 1998 for the 50th annual report of his Harvard College class.

Though Mr. Lee worried mightily about national and world affairs, he focused his efforts locally — including improving a park that was a stroll away from his Beacon Hill home.

“It may be rationalization, but doing something effective in small compass has seemed in many ways more rewarding than grappling futilely with larger issues,” he wrote. “In all events, I have kept busy and out of trouble.”

Bryan Marquard can be reached at bryan.marquard@globe.com.