A versatile character player with a distinctive high, soft, slightly hoarse voice, Kirby is perhaps best remembered as the young Clemenza in Francis Ford Coppola's "The Godfather, Part II" (1974) and the humorless Lt. Hauk in Barry Levinson's "Good Morning, Vietnam" (1987). The son of actor Bruce Kirby, he has been variously billed as Bruce Kirby Jr and B Kirby in his career. Although when younger he was often cast as dweebish characters (as in the sexually-repressed youth in his first film 1973's Harrad Experiment"), as Kirby has matured, he began to play more earthy and street-smart buddies of the star. This was particularly true with both "When Harry Met Sally..." (1989) and "City Slickers" (1991), in which he was Billy Crystal's sidekick. The wiseacre attitude also played well in "The Freshman" (1990); Kirby was the nephew to a reputed gangster don (Marlon Brando) who lures an NYU student (Matthew Broderick) into a scam. More recent roles have included the caring coach of Leonardo DiCaprio in "The Basketball Diaries" (1995), Jason Patric's abusive father in "Sleepers" (1996) and Nicky, the mobster always willing to play the fool, in "Donnie Brasco" (1997).

Since the late 60s, Kirby has been seen on the small screen. He first garnered attention for several appearances as a fast-talking wheeler-dealer student who staged a banana-eating contest on "Room 222" and as an unstable, vengeful killer on an episode of "Kojak". He was Richard Castellano's rude son in the short-lived sitcom "The Super" (ABC, 1972). After nearly a decade absence, Kirby returned to TV in 1992 on "The Larry Sanders Show" (HBO).

Kirby made his Broadway debut replacing Kevin Spacey as a wannabe gangster in Neil Simon's Pulitzer Prize-winning play "Lost in Yonkers" (1991). Six years later, he was back in NYC, this time off-Broadway, playing Alan Zweibel in Zweibel's memoir-play about his friendship with comedienne Gilda Radner, "Bunny Bunny".