He played in Major League Baseball (MLB) for the New York Mets, Los Angeles Dodgers, and Tampa Bay Devil Rays.

[1] Seo attended Gwangju Jeil High School (graduating in 1996),[2] and Inha University in Incheon, South Korea, where he led his team to the Korean collegiate championship in 1997.

On July 21, 2002, Seo made his major league debut with a scoreless inning of relief against the Cincinnati Reds.

Not being able to throw as hard as earlier in his career meant it was important for Seo to maintain a high degree of control over his pitches which, on occasion he was unable to do.

On January 4, 2006, Seo was dealt to the Los Angeles Dodgers along with Tim Hamulack in exchange for relievers Duaner Sánchez and Steve Schmoll.