I started out doing theater and a soap in New York and that's... sort of what I got stuck in. I was blessed enough to have long runs, and it's sort of hard sometimes then to get out.

She got her acting start in the 1960s playing opposite Dustin Hoffman in the Broadway play Jimmy Shine. Sullivan voiced Queen Hippolyta in the animated television series Justice League Unlimited. It was during this time Susan Sullivan played parts in daytime dramas.

She currently appears in Castle as Richard Castle's mother Martha Rodgers. Sullivan played the role of Lenore Curtin Delaney on the daytime soap opera Another World (1971–1976); waitress Lois Adams during the first season (1980-1981) of the comedy It's a Living Maggie Gioberti Channing on the soap opera Falcon Crest (1981–1989); and Kitty Montgomery on sitcom Dharma & Greg (1997–2002).