Mullavey was born in Buffalo, New York, and changed the spelling of his surname from Mulleavy to avoid confusion with his father, who also had the same name.

He played Eddie Gallagher on Rituals, as well as Carly and Spencer's grandfather on iCarly.

He played Biff in the Tales from the Darkside episode (2/12) "Monsters in my Room" (1985).

and Company (1970), Raid on Rommel (1971), The Birdmen (1971), The Love Machine (1971), The Christian Licorice Store (1971), I Dismember Mama (1972), Stand Up and Be Counted (1972), The Single Girls (1974), The Disappearance of Flight 412 (1974), The Hindenburg (1975, as Herbert Morrison), and The Census Taker (1984).

[citation needed] Onstage, he appeared opposite Marlo Thomas in the 2015 New York debut of Joe DiPietro's play Clever Little Lies at the Westside Theatre.