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Born in St. Catharines, Ontario, Slade began his career as an actor with the Garden Center Theatre in Vineland, Ontario. In the mid-1960s, he relocated to Hollywood and began to work as a writer for television sitcoms, including Bewitched. When ABC gave him the opportunity to create a series, he devised Love on a Rooftop, similar in theme to Neil Simon's Barefoot in the Park, about a young couple living in a windowless walk-up apartment with access to a rooftop with a view of San Francisco.
The following year, Slade developed The Flying Nun (adapted from Tere Rios' book, "The Fifteenth Pelican"), with Sally Field as a young novice whose habit's headgear enabled her to fly. He also created The Partridge Family, based on the real-life Cowsills, and Bridget Loves Bernie, inspired by the play Abie's Irish Rose.
Despite his success in television, Slade returned to the theater in 1975 with his play Same Time, Next Year, about a couple who are married to others but meet once-a-year for sex and conversation. With Charles Grodin and Ellen Burstyn in the leads, the play was a major hit and ran for 1453 performances. Slade received the Drama Desk Award and a Tony Award nomination for Best Play. In 1978, he followed with Tribute, the story of a man who learns to love his father, a successful actor who always had more time for his theatrical cohorts than his son. Even with Jack Lemmon heading the cast, it proved to be far less successful than its predecessor, closing after 212 performances. Slightly more successful was Romantic Comedy (1979), starring Anthony Perkins and Mia Farrow. Slade wrote the screenplays for the film versions of all three plays, and was Oscar-nominated for his screen adaptation of Same Time, Next Year.