He has won nine caps for the United States, featuring as backup for Kasey Keller and Tim Howard in two World Cups.

Following his debut for the U.S. in 1994, he played two further games within the same month; however, he had to wait almost nine years before his next international appearance, making him a player with one of the longest-ever gaps between caps.

Hahnemann grew up in the Seattle area in Kent and attended Kentridge High School where he was named the team's MVP his junior season.

He failed to displace Maik Taylor in goal though, and only made four appearances (two in the league) in total for the London club (all during their promotion season of 2000–01).

With Edwin van der Sar signed upon promotion by Fulham, Hahnemann slipped further down the pecking order and to gain playing time, he was loaned out to lower league sides Rochdale,[6] and then, Reading during the 2001–02 campaign.

[11] His first season in the Premier League saw him keep thirteen clean sheets as the club finished eighth, only one place short of European qualification.

He remained with the club for one further season as they attempted an immediate return to the top level, but Reading ultimately lost out to Burnley in the play-off semi finals.

His form ranked him as "the Best Goalkeeper in the World" using the Castrol Performance Index system,[15][16] helping earn him a one-year contract extension.

[24] In 2014, Hahnemann only played two matches for Sounders, both in the U.S. Open Cup, first recording a clean sheet in a 5-0 win over PSA Elite, and then denying the San Jose Earthquakes in penalty kicks after a 1–1 draw on 24 June,[26] and in doing so at the age of 42 years and 10 days, he became the oldest player in the competition's history, a record that has since been broken by Claudio Muñoz in 2024.

[27] Hahnemann made his international debut for the United States national team on November 19, 1994, in a 0–1 friendly defeat to Trinidad and Tobago.

[29] He won two further caps during 2005 and was then picked for the 2006 World Cup in Germany, and allocated shirt number 19,[30] but was an unused substitute in all of the United States' matches behind Kasey Keller and Tim Howard as they exited at the group stage.

[31] Nearing the age of 38, Hahnemann was named by coach Bob Bradley to the United States' 2010 World Cup squad, as third choice behind Howard and Brad Guzan.

[33] Before matches he listens to heavy metal to psych himself up, and presented Five Finger Death Punch with a Wolves shirt emblazoned with a Remembrance Day poppy.

[33] He collaborated with the Reading-based band Malefice in early 2012 to release a song which will debut on Jägermeister UK's Facebook page in February 2012.