The older daughter of a theater-loving mother (who would follow her two children to NYC and found the Thirteenth Street Repertory Company), Jenny O'Hara got her first big break in the touring company for "Brecht on Brecht", appearing with the legendary Lotte Lenya. She made her Broadway debut as one of Dylan Thomas' lovers in the biographical play "Dylan" and her Broadway singing debut in "The Fig Leaves Are Falling", directed by George Abbott, before succeeding younger sister Jill in the role of Fran Kubelik in "Promises, Promises". She also played Sylvie in the female version of Neil Simon's "The Odd Couple" and acted in a revival of Eugene O'Neill's "The Iceman Cometh", among her other Broadway productions.

Primarily a supporting player in TV and movies, although she did star opposite Larry Hagman in NBC's "The Return of the World's Greatest Detective" (1976), O'Hara portrayed Emily Mahoney during the first season (1979-80) of "The Facts of Life" (NBC) and was a series regular as Pam Dawber's loyal assistant on "My Sister Sam" (CBS, 1986-87). Numerous guest spots on series like "Murphy Brown", "Law & Order", "ER" and "The Drew Carey Show" preceded her landing the regular role of Lottie, Sue Murphy's tough, unsentimental working class mother in the short-lived Fox sitcom "Costello" (1998). Among her feature film credits are "Heart Beat" (1979, her debut), "Angie" (1994, with Geena Davis) and "Wes Craven's Wishmaster" (1997). O'Hara also enjoyed a recurring role on CBS' daytime drama "As the World Turns", paired opposite her writer-actor husband Nick Ullett.