Sullivan played the role of Lenore Curtin Delaney on the daytime soap opera, Another World (1971–1976); Maggie Gioberti Channing on the popular 1980s soap opera, Falcon Crest (1981–1989); Kathleen Rachowski on The George Carlin Show; and Kitty Montgomery on the popular 1990s sitcom Dharma and Greg (1997–2002). She voices Fred's Mother in Big Hero 6: The Series. She played Victoria Murray in The Real O'Neals.
Sullivan was born in New York City, but was raised in the Long Island area of Freeport, Nassau County, New York, where she graduated from Freeport High School in 1960.
She earned a BA from Hofstra University in drama in 1964, During her later years in college, she worked as a Playboy Bunny. In 1960, she got her first acting job in a Broadway play Jimmy Shine, with Dustin Hoffman. She got a contract with Universal Studios in 1969.
- Sullivan is currently the only cast member of Castle to not have a Twitter account as of yet, until June 25, 2015, when she got started a Twitter account.
- She was married twice and she is the mother of two sons.
- She has two siblings, a brother named Brendan Sullivan and a sister named Brigid Sullivan.
- She is in a relationship with noted psychologist and author Dr. Connell Cowan.
- She played Dr. Elaina Marks in the pilot of The Incredible Hulk 1978 TV series. Footage from this episode was re-used in the Castle episode "Tick, Tick, Tick...", where it was implied that the part was played by Sullivan's character, Martha Rodgers.