The younger brother of Bob Ammann, he played for the Los Angeles Heat in the Western Soccer League in 1989.
After Mike Stevens, the director of Charlton Athletic, saw Ammann play in the 1994 NCAA Final Four, he offered him a trial.
In 1997, he set a league record (since broken) of twenty-one wins, and ranked fourth in voting for the MLS Goalkeeper of the Year award.
[3] Ammann missed the last five weeks of the 2000 MLS season after Mamadou Diallo collided with him in a game against the Tampa Bay Mutiny.
The incident, which was dubbed "Crime of the Century" by Metro teammate Mike Petke, saw Diallo receive no punishment.
With the emergence of Tim Howard for the MetroStars, Ammann was traded to D.C. United for Richie Williams and an allocation before the 2001 MLS season.