Bernard Slade, a Canadian writer for the stage and screen who created “The Partridge Family” and earned Tony and Oscar nominations for “Same Time, Next Year,” an adulterous romantic comedy that became one of Broadway’s most popular two-handers, died Oct. 30 at his home in Beverly Hills, Calif. He was 89.

The cause was complications from Lewy body dementia, said his literary agent, Charlie Van Nostrand.

Mr. Slade was initially an actor and performed in more than 200 Canadian radio, television and stage productions before turning to writing at age 27, penning witty, romantic teleplays that landed him a job with the production company Screen Gems. He soon dashed off 17 episodes of “Bewitched,” starring Elizabeth Montgomery as a married witch in the suburbs.

Based in Los Angeles, he went on to develop or create a slew of 1960s and ’70s sitcoms, including “Love on a Rooftop,” about newlyweds in San Francisco, and “The Flying Nun,” which starred Sally Field as Sister Bertrille, a novice nun who — through the grace of God and, somehow, the laws of physics — could catch a breeze and fly.

As she explained it, “When lift plus thrust is greater than load plus drag, anything can fly.”

Mr. Slade had considered developing a new sitcom using music when he saw six singing siblings and their mother perform on “The Tonight Show” hosted by Johnny Carson. The group, called the Cowsills, inspired “The Partridge Family,” which premiered on ABC in 1970 and ran for four seasons.

Like “The Monkees,” a popular ’60s series about a manufactured pop band, “The Partridge Family” mixed typical sitcom plots with original musical numbers, including “I Think I Love You” and “Doesn’t Somebody Want to Be Wanted,” largely recorded by professional musicians.

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The series starred Oscar-winning actress Shirley Jones as the mother of five musically gifted children, played by Susan Dey, Suzanne Crough, Jeremy Gelbwaks (later replaced by Brian Forster), Danny Bonaduce and David Cassidy, who became a teen idol as the Partridge Family’s shaggy-haired guitarist, Keith.

Mr. Slade returned to theatrical work after he grew tired of battling with television executives; after one spat, he decided to seek refuge in Hawaii and began writing a play during his flight to Hono­lulu. By the time he landed, he had drafted the first act of “Same Time, Next Year,” which premiered on Broadway in 1975 and ran for 1,453 performances.

The play followed a California housewife and married New Jersey accountant who have a one-night stand at a country inn, then return to renew their affair one weekend each year for two decades. Its structure recalled Jan de Hartog’s Tony-winning 1951 play, “The Fourposter,” which tracked a married couple over 35 years, while “Same Time, Next Year” also touched on the Vietnam War, women’s liberation and America’s shifting social values.

“It is a delicious and very moral kind of immoral play,” wrote New York Times reviewer Clive Barnes. “It has wit, compassion, a sense of humor and a feel for nostalgia — who could ask for anything more? It restores one’s faith in the possibility of a commercially styled Broadway hit — for here is a play clearly geared for popularity that does not for one moment talk down to its audience.”

The Broadway production starred Ellen Burstyn, who won a Tony and later received an Oscar nomination in the film adaptation, and Charles Grodin. He was replaced by Alan Alda in the 1978 film, which earned Mr. Slade an Academy Award nomination for best adapted screenplay. He lost to Oliver Stone for “Midnight Express” and lost the Tony for best play to Peter Shaffer for “Equus.”

Mr. Slade quickly returned to Broadway with two more plays: “Tribute” (1978), which starred Jack Lemmon as a terminally ill press agent and former scriptwriter, and “Romantic Comedy” (1979), featuring Mia Farrow and Anthony Perkins as playwright collaborators who slowly fall in love.

Both were adapted into films and, although they were considered commercial successes, received mixed reviews on Broadway. Not that it mattered to Mr. Slade, who said he was satisfied as long as the audience was laughing.

“Comedy, when done well, looks easy and seems light and frivolous,” he wrote in a 2000 memoir, “Shared Laughter.” “Well, what’s wrong with frivolous? I’ve always believed that laughter is the perfume of life — it makes life bearable. Please . . . send in the clowns.”

Bernard Slade Newbound was born in St. Catharines, Ontario, on May 2, 1930. He was 5 when he moved with his parents to their native England, where his father worked as an airplane mechanic. Moving frequently during World War II, Bernard attended 13 schools in seven years, primarily in the Croydon section of south London.

“Always the ‘new boy,’ both extremely shy and gregarious, I evolved a personality of the class wit,” he wrote in his memoir. He returned to Canada at 18 and began acting, supporting himself with odd jobs that included donning a suit of armor to dress as a knight while handing out leaflets on the streets of Toronto.

“I had no intentions of writing then, nor am I at all sure of how good an actor I was,” he told the Times in 1975. “I realize now that acting is the best training you could possibly have for playwriting. You can relate to the actors’ problems, the realities of staging, the practicality of a scene.”

After turning to television, Mr. Slade created “Bridget Loves Bernie” (1972-73), a CBS sitcom about the marriage between a Catholic woman and Jewish man. They were played by Meredith Baxter and David Birney, who married in real life after the show was canceled, reportedly amid a stream of hate mail to the network protesting its depiction of an interreligious marriage.

His last play on Broadway was “Special Occasions,” which closed after one night in 1982 and tracked a divorced couple through a decade of weddings, funerals and other “special occasions.” Mr. Slade’s other plays included “Fatal Attraction,” his first stab at a thriller (unrelated to the 1987 Hollywood movie), and “Same Time, Another Year,” a sequel to his greatest theatrical hit.

His wife of 64 years, Canadian actress Jill Foster, died in 2017. Survivors include two children, Laurie and Chris Newbound; a sister; and four granddaughters.

Mr. Slade, who had played soccer in England with a man named Partridge, named two of the “Partridge Family” musicians after his own children and named Cassidy’s character after his daughter’s boyfriend at the time. He said he owned about 25 percent of the show’s profits but never saw money from the series, even as it aired in syndication long after being canceled in 1974.

“You know, it’s odd. I wrote television comedy for the money. I wrote the play for myself because I wanted to,” he told an interviewer, referring to “Same Time, Next Year.” “And yet, it’s the play that’s made me the most money. There’s a lesson in that somewhere.”