Kirby was a character actor whose career spanned 35 years. In 1971, he made his screen debut in the dramaThe Young Graduates, although it was his role three years later as the youngPeter Clemenza in the epic crime filmThe Godfather Part II that raised his profile in Hollywood. In the summer of 1972, in one of his early television appearances, Kirby portrayed Anthony Girelli, the son ofRichard Castellano's character Joe Girelli, inThe Super; Castellano had played the older Peter Clemenza inThe Godfather. In 1973, he appeared inThe Harrad Experiment.
His other television appearances includedRoom 222 and thepilot episode ofM*A*S*H, portraying the character Boone (he had no lines). He also appeared in the 1974Columbo episode "By Dawn's Early Light", alongside his fatherBruce Kirby, and in the season 2 episode "Seance" ofEmergency!, where he was credited as "B. Kirby Jr."
Described byLeonard Maltin as the "quintessential New Yorker or cranky straight man", Kirby appeared in a series of comedies, typically playing fast-talking, belligerent yet likable characters. His best-known roles include a colleague ofAlbert Brooks' film editor inModern Romance; a talkative limo driver inThis Is Spinal Tap; the jealous, comedically impaired Lieutenant Hauk inGood Morning, Vietnam; and a shifty assistant toMarlon Brando—a parody of Brando'sGodfather role—inThe Freshman. Kirby balanced comedies with dramatic roles likeDonnie Brasco as a double-dealing mobster.
During the 1970s Kirby was in a relationship with actressAnnette O'Toole.[5] He married actress Lynn Sellers on September 29, 2003. They remained married until his death in 2006.[citation needed]