In 1984, Zimmerman was drafted in the second round (36th overall) by the Los Angeles Express in the 1984 USFL Collegiate Draft. He subsequently signed with the Express on February 13, 1984 and went onto play in 17 games that season, starting all 17 at left tackle. His Express teammate was future Pro Football Hall of Famer, QB Steve Young. The Express lost in the USFL Semi-Finals to the Arizona Wranglers to end a 10-8 regular season. In 1985, Zimmerman suited up again with the Express, playing in 18 games (starting 17) with a 3-15 club that ended up out of the USFL playoff picture.
During his time in the NFL, Zimmerman was famous for his refusal to interact with the media. This disdain for the sports press came about due to an early incident in his NFL career, after comments made by Zimmerman condemning the Vikings offensive players for a loss were made public by the media. Zimmerman claimed that his teammates ostracized him for speaking ill of his teammates performance; this led Zimmerman to decide to boycott the sports media as a result, refusing to do interviews or engage in any sort of interaction with them for the rest of his career.
Zimmerman ultimately left the Vikings for the Broncos in 1993, and stayed with the team from 1993-1997. He would be part of the team's first Super Bowl-winning squad, winning the game in 1997. Arriving as the veteran player in 1993 to an offense that was made up of mostly rookies, Zimmerman became the de facto leader of the Broncos offensive line; off the field, Zimmerman forced his fellow players to join him in shunning the media, a policy that would remain in effect until his retirement in 1997.
He played in 184 NFL games, starting 169 of them.