While working with Robert Altman, Tewkesbury co-wrote the screenplay for Thieves Like Us and wrote Nashville, for which she was nominated for a WGA Award, a Golden Globe and a BAFTA Award, and won the L.A. Film Critics Award for Best Screenplay.
Making her directorial debut in 1979 with the feature film Old Boyfriends, she went on to write and direct movies for television such as The Acorn People, Cold Sassy Tree, Scattering Dad and Sudie and Simpson. Tewkesbury has also directed television series like Northern Exposure; Doogie Howser M.D., Felicity, Picket Fences, and The Guardian, which she also executive produced.
Joan Tewkesbury served as a member of the Western Directors Council from 1999 to 2001 and also served on the DGA's Movies for Television Committee.