Before Harley Quinn he was well-known for voicing the character of Darkseid in the DC Animated Universe, with him first appearing in the role in 1997 on Superman: The Animated Series. Ironside also stared as the 80s Batman in the The New Batman Adventures episode "Legends of the Dark Knight". He also voiced the character in the 2018 video game, Lego DC Super-Villains and has appeared in live-action DC properties like The Flash and Smallville.
He has worked as a voice actor, producer, film director, and screenwriter in film and television series in various Canadian and American productions. He is best known for playing villains and "tough guy" heroes, though he has also portrayed sympathetic characters. He is, among other roles, known for his roles in Starship Troopers and Total Recall and appearing the TV show V during the 1980s. His first major breakout performance in film came in the form of the psychokinetic villain Darryl Revok from the science-fiction horror film Scanners.