A veteran character actor, he is widely known for his 1993 Emmy Award-nominated performance on Seinfeld, portraying the embittered, irascible retiree Sid Fields.

He had credited small roles in films such as The Christine Jorgensen Story (1970), How Awful About Allan (1970), Candy Stripe Nurses (1974) and Tarantulas: The Deadly Cargo (1977), before he co-starred alongside Christopher Reeve and Jane Seymour in the romantic fantasy Somewhere in Time (1980) as Arthur Biehl, the Grand Hotel's venerable bellman, and attended annual reunions of cast, crew, and fans of the cult classic at the Grand Hotel on Mackinac Island, Michigan.

Erwin also appeared in numerous films by John Hughes, with cameos in Planes, Trains and Automobiles (1987), She's Having a Baby (1988), Home Alone (1990) and Dennis the Menace (1993).

His later film career included roles in Invitation to Hell (1984), The Land Before Time (1988), Naked Gun 33+1⁄3: The Final Insult (1994), Things to Do in Denver When You're Dead (1995), Menno's Mind (1997), Chairman of the Board (1998), Forces of Nature (1999), Inferno (1999) and A Crack in the Floor (2001).

In the 1960s, Erwin appeared in television series such as: The Andy Griffith Show, Mister Ed, Maverick, The Twilight Zone, 87th Precinct, My Three Sons, The Fugitive, Leave It to Beaver and Mannix.

In the 1970s, 1980s and 1990s, he appeared in Barnaby Jones, Cannon, Gunsmoke, Married... with Children, E/R, The Optimist, Highway to Heaven, Who's the Boss?, Growing Pains, Full House, The Golden Girls, Moonlighting, Star Trek: The Next Generation, Lois & Clark: The New Adventures of Superman and The Drew Carey Show.