Haggis began to work as a writer for television programs, including The Love Boat, One Day at a Time, Diff'rent Strokes, and The Facts of Life. With The Facts of Life, Haggis also gained his first credit as producer. During the 1980s and 1990s, Haggis wrote for television series including thirtysomething, The Tracey Ullman Show, FM, Due South, L.A. Law, and EZ Streets. He helped to create the television series Walker, Texas Ranger; Family Law; and Due South. Haggis served as executive producer of the series Michael Hayes and Family Law.

He gained recognition in the film industry for his work on the 2004 film Million Dollar Baby, which Allmovie described as a "serious milestone" for the writer/producer, and as "his first high-profile foray into feature film". Haggis had read two stories written by Jerry Boyd, a boxing trainer who wrote under the name of F.X. Toole.