Gerety was born in Providence, Rhode Island. He 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 resident theater company in Providence, where he appeared in over 125 productions.

In the late 1990s, he joined the cast of the Barry Levinson produced NBC police drama Homicide: Life on the Street. He was a regular guest star on Law & Order and HBO's The Wire. He played FBI Agt. Franklin Morgan in the short-lived American adaptation of Life on Mars. In the final season of Brotherhood as Martin Kilpatrick. More recently, he was seen on Mercy and Brothers and Sisters. He appears in the 2011 NBC series Prime Suspect as Maria Bello's father.

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 Hothouse. He has since performed in many more plays both on and off-Broadway, most recently in Martin McDonagh's The Lieutenant of Inishmore.

He also appeared in such feature films as K-Pax, People I Know, The Curse of the Jade Scorpion, Hollywood Ending, Wolf, Charlie Wilson's War, Surviving Picasso, War of the Worlds, Things That Hang from Trees, Inside Man, and Changeling. He played Louis Piquett in Public Enemies (2009 film, directed by Michael Mann, and had a small part in Mel Gibson's Get the Gringo (2011). In 2013, Gerety was playing the role of John Cotter in Nora Ephron's Broadway play Lucky Guy with Tom Hanks. In 2017, Gerety plays Otto Bernhardt, the patriarch of the new Amazon series Sneaky Pete.