“I had a whole goddamned career of ‘Yassuh, can I git ya another drink,’ ” Sidney said. Photograph from Schomburg Center / New York Public Library

Racial diversity on television is in a state of rapid acceleration. In 2012, when “Scandal” débuted, starring Kerry Washington as a Capitol Hill fixer, it was the first network drama to feature a black female lead in thirty-eight years—a shameful milestone. The same fall, “The Mindy Project,” on Fox, made a brown girl the madcap heroine of a sitcom, not her best friend. Just three years later, “Scandal” faces off with “Empire”; “Black-ish” and “Fresh Off the Boat” have helped rebrand ABC as “the diversity network”; Aziz Ansari’s “Master of None” struts on Netflix; the Latina-centric “Jane the Virgin” lights up the CW; and Priyanka Chopra plays the lead on “Quantico.” There has been an especially remarkable migration of black actresses from movies to TV, among them Taraji P. Henson, Viola Davis, Angela Bassett, Gabourey Sidibe, Lorraine Toussaint, and Gabrielle Union. There is also a deluge of new talent on shows like Netflix’s “Orange Is the New Black,” one of several series that have opened the floodgates for performers who were long denied rich, complex central roles.

Hollywood, television included, is still run by white decision-makers, mostly men. The recent season of “Project Greenlight,” on HBO, made explicit how resistant to race talk Hollywood can be, a stifling culture of bros bonding with mirror versions of themselves. Behind-the-scenes numbers have barely shifted, particularly for directors. And yet TV is evolving rapidly. Much of this is due to a prominent new set of creative figures, among them Ansari and Kaling, Shonda Rhimes and Kenya Barris, Lee Daniels and Larry Wilmore, Nahnatchka Khan and John Ridley, Dee Rees and Mara Brock Akil, who don’t merely perform but run the show. Even newer is the increasing bluntness of many creators. When Viola Davis won an Emmy for Best Actress, for ABC’s “How to Get Away with Murder,” she gave a bold and unapologetic speech in which she quoted Harriet Tubman and declared, “The only thing that separates women of color from anyone else is opportunity. You can’t win an Emmy for roles that are simply not there.”

This is thrilling and long overdue. But it’s also a phenomenon that could easily recede, as it has many times before after periods of progress: in the early fifties, when television was brand-new; in the seventies, the era of “Roots” and Norman Lear; and again in the early nineties, post-Cosby, when black sitcoms thrived. One observer understood this ephemeral quality more than most: P. Jay Sidney, an African-American actor who built a four-decade career in television, all the while protesting network racism, in what Donald Bogle’s book “Primetime Blues” recounts as a “one-man crusade to get African-Americans fair representation in television programs and commercials.” Sidney is a footnote in history books, while other activists of his era are heroes. But he was there when the medium began, appearing on TV more than any other black dramatic actor of the time. Even as his résumé grew, Sidney picketed, he wrote letters, he advocated boycotts, he taped interactions with executives, lobbying tirelessly against TV’s de-facto segregation. In 1962, he testified before the House of Representatives. Nothing made much headway; he grew disgusted and disaffected. By the time Sidney died, in Brooklyn, in 1996, he had largely been forgotten, a proud loner who never got to see his vision become reality. “People today benefit from things that were sacrificed years ago,” his ex-wife Carol Foster Sidney, who is now eighty-seven, told me. “And they haven’t a clue.”

Sidney was born Sidney Parhm, Jr., in 1915 in Norfolk, Virginia, and grew up in poverty, in an era of public lynchings and Jim Crow. His mother died when he was a child; his father moved the family to New York, then died when his son was fifteen. According to a 1955 profile, titled “Get P. Jay Sidney for the Part,” he was a “difficult” child who landed in foster care but excelled academically—he graduated from high school at fifteen, then went to City College for two years, dropping out to enter the theatre. A lifelong autodidact, he is described by those who knew him as a guarded, sardonic figure, eternally testing those around him against an intellectual ideal. But even during the Depression he got jobs: he was in Lena Horne’s first stage play, in 1934; in the forties, he appeared in “Carmen Jones” and “Othello.” In a photograph taken at a campaign event for Franklin D. Roosevelt, Sidney is a dapper bohemian with a clipped beard. He also built a radio career, producing a series called “Experimental Theatre of the Air,” which, in a radical move, cast voices without regard to racial categories. Sidney collected his press clippings in a binder, which is saved at the New York Public Library’s Schomburg Center.

As the country came out of the Depression, and the civil-rights movement began, progress for black actors may have seemed possible. When television emerged, in the forties, it was a low-status but experimental medium, suggesting tantalizing opportunities for innovators. Yet a newspaper article from the mid-fifties, headlined “TV’s New Policy for Negroes,” depicts Sidney as the “single exception” to the exclusion of black dramatic actors. In TV’s infancy, the article laments, “The video floodgates were expected to be thrown open to experienced Negro actors. It never happened.”

“We took it for granted that we would be the last hired if hired at all and the first fired,” Ossie Davis recalled, in “The Box,” Jeff Kisseloff’s oral history of television. “And that we would wind up doing the same stereotypical crap that we did on Broadway.” “Amos and Andy” was typical fare. In the late fifties, Davis participated in a TV boycott in Harlem, in which black viewers turned off their sets one Saturday night. But it was Sidney’s rabble-rousing that had a direct influence on Davis’s career: “He used to walk around with a sign, accusing the broadcast industry of discriminating against black folks. As a response to P. Jay’s accusations, CBS didn’t give him a job, but they gave me one.”

From 1951 on, Sidney made a living on TV, getting a few notable roles, including Cato, Hercules Mulligan’s slave and fellow-spy, in “The Plot to Kidnap General Washington,” in 1952. For two years, he appeared as one of two African-American soldiers on “The Phil Silvers Show”—a casting move protested by Southern stations. (The writers ignored them.) Over time, he amassed roles on more than a hundred and seventy shows, as well as a lucrative sideline in voice-over work and advertisements. (He played the onscreen role of Waxin Jackson for Ajax.) But the majority of his parts were walk-ons: doormen, porters, waiters. “I had a whole goddamned career of ‘Yassuh, can I git ya another drink, sir?,’ ” he told Kisseloff. “But I did what was available. I did not mix feelings with the fact that I needed money to live.”

With each setback, Sidney grew more frustrated, according to Foster Sidney, who married Sidney in 1954. Foster Sidney was the daughter of a dentist, educated at Howard University, a member of the Washington, D.C., African-American élite. She had persuaded her family to let her move to New York to be a French translator but dreamed of being an actress. Foster Sidney recalls, “He knew I had these aspirations, but he said, ‘One actor in the family.’ I, timid little thing, said, ‘Yes, dear.’ ” Their marriage was contentious, with Sidney resenting Foster Sidney’s “bourgeois” background; they separated, and had no children, but did not divorce until 1977. (In later years, Foster Sidney returned to acting, a period she calls “ten years in Heaven.”)