After briefly leaving Screen Gems to work as a script supervisor on The Courtship of Eddie's Father for Metro-Goldwyn-Mayer, he came back to Screen Gems to create The Partridge Family, based on the real-life Cowsills, and Bridget Loves Bernie, inspired by the play Abie's Irish Rose.
He also wrote the script to the 1972 Columbia Pictures film Stand Up and Be Counted, directed by Jackie Cooper and starring Jacqueline Bisset, in which the Helen Reddy song "I Am Woman" was first introduced.
Despite his success in television, Slade returned to the theater in 1975 with his play Same Time, Next Year,[5] about a couple who are married to others but meet once-a-year for sex and conversation.
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.
Slade wrote the screenplays for the film versions of all three plays, and was nominated for an Oscar for his screen adaptation of Same Time, Next Year.