On the second day of the annual convention of the American Psychiatric Association in 1972, something extraordinary happened.

While the assembled psychiatrists, mostly white men in dark suits, settled into rows of chairs in the Danish Room at the Adolphus Hotel in Dallas, a disguised figure had been smuggled through the back corridors. At the last minute, he stepped through a side curtain and took his place at the front of the room.

There was an intake of breath in the audience. The man’s appearance was grotesque. His face was covered by a rubber Nixon mask, and he was wearing a garish, oversized tuxedo and a curly fright wig. But the outlandishness of his outfit diminished in importance once he began to speak.

“I am a homosexual,” he began. “I am a psychiatrist.”

For the next 10 minutes, Henry Anonymous, M.D. — this is what he had asked to be called — described the secret world of gay psychiatrists. Officially, they did not exist; homosexuality was categorized as a mental illness, so acknowledging it would result in the revocation of one’s medical license, and the loss of a career. In 42 states, sodomy was a crime.

The reality was that there were plenty of gay people in the A.P.A., psychiatry’s most influential professional body, the masked doctor explained. But they lived in hiding, concealing every trace of their private life from their colleagues.

“All of us have something to lose,” he said. “We may not be under consideration for a professorship; the analyst down the street may stop referring us his overflow; our supervisor may ask us to take a leave of absence.”

This was the trade-off that had formed the basis of the masked man’s life. But the cost was too high. That’s what he had come to tell them.

“We are taking an even bigger risk, however, in not living fully our humanity,” he said. “This is the greatest loss, our honest humanity.”

He took his seat to a standing ovation.

The 10-minute speech, delivered 50 years ago Monday, was a tipping point in the history of gay rights. The following year, the A.P.A. announced that it would reverse its nearly century-old position, declaring that homosexuality was not a mental disorder.

It is rare for psychiatrists to transform the culture that surrounds them, but that is what happened in 1973.

By removing the diagnosis from the Diagnostic and Statistical Manual of Mental Disorders, or D.S.M., psychiatry removed the legal basis for a wide range of discriminatory practices: for denying gay people the right to employment, citizenship, housing and the custody of children; for excluding them from the clergy and the military and the institution of marriage. The long process of rolling back those practices could begin.

When referred to psychiatrists, gay people would no longer be sent to be “cured” — injected with hormones, subjected to aversion therapy or pored over by analysts — but instead told that, from the point of view of science, there was nothing intrinsically wrong with them.

After delivering his speech, the man in the mask, John Ercel Fryer, 34, flew from Dallas to his home in Philadelphia, noting in his journal just how terrifying and profound the experience had been.

“The day has passed, it has come and gone and I am still alive. For the first time I have identified with a force that is akin to my selfhood,” he wrote, in excerpts included in “Cured,” a 2018 documentary.

Still — he didn’t tell his mother he had done it. He didn’t tell his sister. He didn’t tell his closest childhood friend. He barely told anybody for 20 years.

Dr. Fryer, who died in 2003 at the age of 65, stood out for his size (he was 6-foot-4 and 300 pounds), for his flashing intelligence, and for the fact that he was obviously gay.

Betty Lollis, a friend from Winchester, Ky., recalled him as the round-faced boy who was led into her second-grade class, dressed by his mother in a sailor suit. He was a prodigy, she said, and also “just a boy the boys laughed at or teased.”

Decades later, Ms. Lollis said, some of their classmates apologized to Dr. Fryer for the way they had treated him. “These people that were painful for him were also all he had,” she said. “Those are his dearest friends.”

He sailed through his classes, enrolling in college at 15 and medical school at 19. But again and again, his path was blocked when supervisors learned he was gay.

The most crushing of these setbacks occurred in 1964. He had relocated to the freer atmosphere of the East Coast, and was a few months into a residency at the University of Pennsylvania when he let his guard down, telling a family friend at dinner that he was gay.

The young man immediately reported this to his father, who reported it to the department chairman at Penn, Dr. Fryer said in a 2002 interview with the Journal of Gay and Lesbian Psychiatry. The department chairman called Dr. Fryer into his office and said: “You can either resign or I’ll fire you.”

It took years of humiliating assignments at a state-run psychiatric hospital, the only institution that accepted him, for Dr. Fryer to complete his residency. After that he faced a long, uncertain path to tenure. For these reasons, coming out had little appeal, he said in a 2001 interview for “This American Life,” much of which has not been published until now.

“It was a way, if you came out as gay, to not have any power,” he said. “And I wanted to be powerful. So being a straight, closeted physician enabled me to have power.”

In 1970, Frank Kameny, an astronomer who had been dismissed from the military because he was gay, led a small group of gay rights activists to protest the A.P.A.’s annual convention, demanding that the diagnosis be declassified.

Dr. Fryer was a full-fledged member of the “Gay P.A.,” a group of closeted A.P.A. members. who gathered in secret on the edges of the association, and he watched with distaste as the protesters stormed into panel discussions and heckled the speakers. “I was embarrassed by it, and I wished that they would shut up,” he said.

But the following year, Barbara Gittings, one of the activists, approached Dr. Fryer to ask for his help.

Younger, more progressive leaders were rising through the ranks of the A.P.A., and the activists sensed an opening. They had an idea: Instead of picketing, they could shake things up by confronting the psychiatrists with one of their own, a gay psychiatrist. If only they could find someone who would agree to do it.

“My first reaction was: No way,” Dr. Fryer recalled. “I had no security, and I did not want to do anything to jeopardize the possibility that I could get a faculty position somewhere. There was no way at that point that I was going to do that as an open thing.”

Over the months that followed, though, Ms. Gittings kept calling. She updated Dr. Fryer as she approached a dozen of his gay colleagues and each said no, the risk was too great.

Their refusals bothered Dr. Fryer. And Ms. Gittings, as he put it, kept “upping the ante.” What if she paid his way to Dallas? What if he wore a disguise, so that no one knew it was him?

“She planted in my mind the possibility that I could do something,” he said. “And that I could do something that would be helpful without ruining my career.”

Dr. Fryer’s lover at the time was a drama student, and the two threw themselves into the project of devising a disguise that would conceal his identity: a vastly oversized tuxedo, a rubber mask melted to distort its features, and a wig with a low hairline opposite to his own.

Stepping onto the stage that day, Dr. Fryer said, “I felt a great freedom, a great sense of freedom.”

There was pride, too, that he was the only one of his colleagues who dared.

“To do that thing, to be willing to do that thing, when none of my colleagues in the Gay P.A. would be wiling to do it, openly or otherwise,” he said. “They were all in the audience. They were clapping.”

The sight of Dr. Fryer had a powerful emotional effect on the psychiatrists gathered in the room, said Dr. Saul Levin, who in 2013 became the first openly gay man to serve as the A.P.A.’s chief executive and medical director.

“It obviously really shook them,” he said. “Here was this huge audience for the time, seeing someone come out in a very weird costume. It made them a little disoriented — what the hell is going on here? And then this person comes out with such an eloquent speech.”

Dr. Fryer was giddy as he left the stage, so exhilarated that, before returning to Philadelphia, he splurged on a manual harpsichord, which he wryly described as “among the least wise choices of my life.”

As he returned to his hotel room to change out of his disguise, he passed the chairman of the psychiatry department at the University of Pennsylvania, who had fired him from his residency. Neither man showed any sign of recognition.

‘It was over for me’

Dr. Fryer returned to the rambling, Victorian house where he lived in Germantown with his Doberman pinschers and the medical students he took in as boarders.

He remained himself — by turns generous and overbearing, charismatic and acerbic, switching on his Kentucky accent when it suited him.

He still didn’t have tenure, and his career path was as tenuous as ever. In 1973, the A.P.A. voted to declassify homosexuality. And Dr. Fryer lost another job, this one at Friends Hospital.

Again, an administrator called him into his office. “If you were gay and not flamboyant, we would keep you,” Dr. Fryer recalled him saying. “If you were flamboyant and not gay, we would keep you. But since you are both gay and flamboyant, we cannot keep you.”

Dr. Fryer watched as his colleagues got promoted and won tenure. The Gay P.A. faded, as a new, more activist generation stepped forward as an open force within psychiatry, forming the Association of Gay and Lesbian Psychiatrists. But Dr. Fryer took no part in it.

“I ran away again,” he said. “I didn’t go to the meetings. It was like I just sort of disappeared.” It was as if, he said, “I had done my thing and it was over for me.”

Every now and then, he would tell someone about what he had done.

Dr. Karen Kelly, 67, who rented a room from Dr. Fryer as a medical student, said he told her over dinner some time in the late 1970s, and never mentioned it again.

Ms. Lollis, 85, said she and Dr. Fryer confided in one another later in life, sometimes speaking on the phone several times a week. But she didn’t find out that he was Dr. Anonymous until 2002, when he sent her the episode of “This American Life” that described the speech.

“He just didn’t share it with anyone,” she said. “Not his mother, not his sister.”

Dr. Fryer would eventually get tenure at Temple University, where he built a specialty in bereavement and helped pioneer the hospice movement. After teaching all day and having dinner, he would often see patients until 11 p.m., Dr. Kelly recalled. He sat with many of his patients while they were dying.

He threw big parties, and sometimes his famous friends, like the anthropologist Margaret Mead or the writer Gail Sheehy, would show up. He wore dashikis. Traveling for conferences, “he’d end up in a tiki restaurant with my cousins, dancing with the hula dancer,” Dr. Kelly said.

But a sense of resentment clung to him, said Dr. David Scasta, who got to know Dr. Fryer as a medical resident at Temple University and interviewed him about his life in 2002.

He felt isolated from the gay community, said Dr. Scasta, a past president of the Association of Gay and Lesbian Psychiatrists. He never had a long-term relationship. And he always felt that his career was not what it could have been.

“There was always a sense of sadness at not being fully accepted,” he said. “John always felt he was on the fringe.”

Decades would pass before historians of gay rights fully understood the significance of the Dr. Anonymous speech, that it had “a Stonewall riots kind of importance,” Dr. Scasta added. In that case, too, the surge of forward motion was driven by unlikely people.

“It’s not always the law-abiding, nice people who did it, it’s the ones who are on the periphery who can make change,” he said.

On Monday, the 50th anniversary of the Dr. Anonymous speech will be celebrated with speeches and proclamations in Philadelphia, which has declared May 2 John Fryer Day.

Public celebration of his act had already begun in the years before Dr. Fryer’s death, and in 2001 he remarked on it caustically, saying he “sort of was trundled out as an exhibit every time someone wanted an exhibit.”

At the time, though, it was secrecy that gave his act its power, he said.

“As this person who was in disguise, I could say whatever I wanted,” he said, adding, “I did this one isolated event, which changed my life, which helped change the culture in my profession, and I disappeared.”