As a freshman at Maryland, Seitz won the NCAA national championship by posting a 13–1–2 record with a 0.89 goals against average.

[citation needed] Seitz was drafted in the 2007 MLS SuperDraft where he was selected fourth overall by Real Salt Lake.

[citation needed] In three seasons in Salt Lake, Seitz was never able to un-seat veteran Nick Rimando as RSL's first-choice keeper.

[6]Seitz spent the first 22 games of Philadelphia's inaugural 2010 season as the Union's starter, before being dropped to the bench behind Brad Knighton.

A contract was agreed with Seattle, and he was immediately traded to FC Dallas in exchange for a fourth-round pick in the 2012 MLS SuperDraft.

[10] In 2012, due to an injury to Kevin Hartman, who was dealing with back spasms, Seitz started against Vancouver Whitecaps FC in a 0–1 loss for Dallas.

[13] Seitz continued to be primarily a backup goalkeeper for Dallas until the 2016 season, when he would start a career-high 28 MLS matches.

[13] He would contribute to the team winning two major trophies: The Supporters' Shield and the Lamar Hunt U.S. Open Cup.

[18] Seitz appeared in 6 of D.C. United's 23 games in a shortened 2020 season due to the COVID-19 pandemic, keeping 2 clean sheets.