While putting herself through college at Hofstra University, she worked as a Playboy bunny[citation needed]. She got her start in acting in the 1960s, playing opposite Dustin Hoffman in the Broadway play Jimmy Shine. She also landed a contract with Universal Studios in 1969, guest-starring on several shows. It was at this time, she played parts in daytime dramas. Starting with a role on A World Apart in 1970, she moved to a five year stint as Lenore Moore Curtin Delaney on the daytime soap opera Another World. Taking over the role from Judith Barcroft, Sullivan played the role from 1971 to 1976, and it was this role that first established her as a household name. Sullivan was tapped to play Charlton Heston's girlfriend Ann, a major new character written into the TV version of the film Midway in 1976. She also played Dr. Elaina Marks opposite Bill Bixby in the Incredible Hulk TV Series pilot in 1977 and portrayed Poker Alice Ivers opposite James Garner as Bret Maverick in The New Maverick the following year.

An Emmy nomination for her role as Maggie Porter in the television drama Rich Man, Poor Man Book II brought her even more attention. She also played Dr. Julie Farr in Having Babies, which was made into two television movies and later a short-lived weekly series.

In 1980, she appeared on the sitcom It's a Living, playing head waitress, Lois Adams.

Sullivan has never married or had children, but her boyfriend for over fifteen years has been psychologist and author, Connell Cowan. She also previously dated, among others, screen icon Cary Grant.