On television, he appeared in guest turns on The Rockford Files, Happy Days, Hawaii Five-O, Dallas, All in the Family, Soap, Good Times, The Waltons, Hart to Hart, Trapper John, M.D., M*A*S*H, Hill Street Blues, Lou Grant, The Equalizer, Wings, Picket Fences, In the Heat of the Night, Law & Order, Mad About You, Murder, She Wrote, JAG, NYPD Blue, and Coach, among many others.

An accomplished stage veteran, Hallahan was thrice honored with Los Angeles Drama Critics Circle Awards for Best Lead Performance for his turns in productions of Rat in the Skull in 1986, The Kentucky Cycle in 1992, and Endgame in 1995.

[1] Hallalhan was a member of the American Conservatory Theater and appeared in many productions there such as One Flew Over the Cuckoo’s Nest, Equus, and The Threepenny Opera.

[1] Hallahan also co-starred as a regular cast member in two popular television series, Grace Under Fire[2] and the long-running The Paper Chase.

[1] His final screen turn--and his lone big-screen leading role--was as corrupt Los Angeles homicide detective Jack Stillman in the 2001 independent Hitchcockian noir-thriller Mind Rage, a film eventually released, posthumously, four years after his untimely death.

In 1997, Hallahan was cast as Liam Bilby for the Star Trek: Deep Space Nine episode "Honor Among Thieves", but was replaced after having passed away.