Gerety began acting while a student at Boston University, participating in productions at the Charles Playhouse. In 1965, he joined the Trinity Square Repertory Company, a theater troupe in Providence, Rhode Island where he appeared in over 125 productions. Gerety is a veteran of stage, screen and television. In early 1992, he performed to critical acclaim on Broadway in Conversations with My Father, starring Judd Hirsch, and in Harold Pinter's The Hothouse. In the late 90s he joined the cast of the Barry Levinson produced NBC police drama Homicide: Life on the Street. Gerety has been a regular guest star on Law & Order and HBO’s The Wire as Judge Daniel Phelan and has appeared in numerous feature films with the likes of Jack Nicholson, Shirley MacLaine, Jeff Bridges, and Kevin Spacey; and with directors Woody Allen (The Curse of the Jade Scorpion, Hollywood Ending), Mike Nichols (Wolf), Barry Levinson (Sleepers), James Ivory (Surviving Picasso), Steven Spielberg (War of the Worlds), and Ido Mizrahy (Things That Hang from Trees).