She gained further critical acclaim for her role as Captain Doreen Lewis in Private Benjamin, earning an Oscar nomination for Best Supporting Actress.

[2][3][4][5] After graduating from high school in California, Brennan moved to Washington, D.C., to attend Georgetown University, where she was a member of the Mask and Bauble Society.

Her exceptional comic skills and romantic soprano voice propelled her from unknown to star in the title role of Rick Besoyan's off-Broadway tongue-in-cheek musical/operetta Little Mary Sunshine (1959),[7] earning Brennan an Obie Award, and its unofficial sequel The Student Gypsy (1963), on Broadway.

[13] In 1972, Brennan appeared in an All in the Family episode, "The Elevator Story" (1972), as Angelique McCarthy, followed by a role in George Roy Hill's Academy Award-winning film The Sting (1973) as Billie, the brothel madam and confidante of con man Henry Gondorf (Paul Newman).

Bogdanovich was the only director who made use of her musical talents (save for The Cheap Detective; and she sang in performances off Broadway) when he cast her as Cybill Shepherd's crude, fun-loving maid in his musical flop At Long Last Love (1975) (which also starred Madeline Kahn; Brennan and Kahn worked together in two more films: The Cheap Detective and Clue).

In 1980, Brennan received a Best Supporting Actress Oscar nomination for her role as Goldie Hawn's nasty commanding officer in Private Benjamin.

[14] In 2001, she made a brief appearance in the horror movie Jeepers Creepers, and the following year starred in the dark comedy film Comic Book Villains, with DJ Qualls.

In recent years, Brennan had guest-starred in television, including recurring roles as the nosy Mrs. Bink on 7th Heaven and as gruff-acting coach Zandra on Will & Grace.

In 2003, director Shawn Levy cast her in a cameo role of a babysitter to Steve Martin and Bonnie Hunt's children in an updated remake of Cheaper by the Dozen.

From 1968 to 1974, Brennan was married to British poet and photographer David John Lampson, with whom she had two sons: Patrick, a former basketball player turned actor, and Sam, a singer.

[7] In 1982, Brennan was hit by a passing car in Venice Beach while leaving a restaurant with Goldie Hawn and suffered massive injuries.