Ahrens was selected by the St. Louis Cardinals in the sixth round of the 1981 NFL draft. He recorded his only interception his rookie year, which was returned for a touchdown. Ahrens played in all the games and started ten his rookie season, but his playing time diminished and after four years he was traded to the Indianapolis Colts for a tenth-round pick in the 1986 NFL draft.
Ahrens was traded to the Indianapolis Colts before the 1985 season. He did not start a single game in 1985 but had arguably his best NFL season in 1986, starting ten games, and registering two quarterback sacks, one of them helping the Colts secure their first win of the year late in the season against the Atlanta Falcons.
Detroit Lions
Ahrens spent 1988 with the Detroit Lions but only played in half the games and was left unprotected.
Miami Dolphins
Ahrens was signed by the Miami Dolphins before 1989 and recorded one sack with the team. Due to salary concerns, the Dolphins released him but he did sign a Plan B contract.
Ahrens spent the 1991 season with the Colts as an assistant coach. He was part of a large group of Colts players named in a 2011 concussion lawsuit against the NFL.