Harris' film career started in the 1980s when he struck a friendship with Herschel Weingrod. With Weingrod, Harris collaborated on several projects writing stories and screenplays for films such as Trading Places, Brewster's Millions, Twins, Kindergarten Cop and Space Jam. The pair also produced Joel Schumacher's 1993 film Falling Down.
In reviewing Woody Allen's film Zelig for The New York Times in July of 1983, film critic Vincent Canby said "Trading Places is one of the best American comedies in a long time, but I'd be hard-put to assign more responsibility for the film's success to its director, John Landis, than to Timothy Harris and Herschel Weingrod, who wrote it".
For his work in film, Harris has been nominated for a BAFTA Award for Best Original Screenplay as well as an Annie Award for Writing in a Feature Production.