A veteran of film and television, Erwin appeared on numerous "theater programs" of the early 1950s, including Science Fiction Theater and Zane Grey Theatre. He has made guest appearances on a wide variety of television programs, including The Virginian, Gunsmoke, Growing Pains, Who's the Boss?, Seinfeld (for which he received an Emmy nomination, with Lanei Chapman playing his housekeeper), The Drew Carey Show, The West Wing, I Love Lucy, and The Twilight Zone. In the latter, he co-starred with fellow Trek alumni Ken Lynch, Arthur Batanides, Frank Overton, John Hoyt, and Morgan Jones.
His film credits consist primarily of small roles in such films as Forces of Nature, Naked Gun 33 1/3: The Final Insult (with Marc Alaimo), Home Alone, She's Having a Baby (with William Windom and Wil Wheaton), and The Bear. The exception to this is Somewhere in Time, where he was featured in a larger role alongside Christopher Reeve, Jane Seymour, and Star Trek VI: The Undiscovered Country actor Christopher Plummer.
He had roles in two films with Star Trek III: The Search for Spock actor Christopher Lloyd: Dennis the Menace (with Paul Winfield) and Things to Do in Denver When You're Dead (with Bill Cobbs, Don Stark, Tommy "Tiny" Lister, Jr., and Bill Bolender). Irwin also appeared in the HBO movie Menno's Mind, starring Michael Dorn, Robert Picardo, and Corbin Bernsen.
He also appeared on an episode of Monk, with James W. Jansen.