Hallahan was born in Philadelphia and graduated from Rutgers University, then going on to Temple University to earn a master's degree in fine arts. During his acting career he was often cast as a police officer, and may have been best known as LAPD Captain Charlie Devane on Hunter. He was memorable for his portrayal of the Nameless "Coach" in Vision Quest, opposite Matthew Modine. He also served in the US Navy in the early 1960s, including time as a Navy hospital corpsman stationed in Puerto Rico.
In 1997, Hallahan was cast as Liam Bilby in the Star Trek: Deep Space Nine episode "Honor Among Thieves". On November 25, 1997, he died of an apparent heart attack while driving his car in Los Angeles. Nick Tate assumed the role of Liam Bilby in Hallahan's memory. Hallahan is buried in Cobh, Ireland, where he had family roots.
