He was signed by the Seattle Mariners as an amateur free agent on September 25, 1998.

He carried a no-hitter into the eighth inning, retiring 21 of the first 22 batters faced, before allowing a single, on July 23.

He made his MLB debut on August 8, 2004 against the Tampa Bay Devil Rays.

Baek was non-tendered by the Mariners in December 2005 following a disappointing year in Triple-A with the Tacoma Rainiers.

He posted an 11–4 record with a 2.80 ERA, which earned him a recall to the Mariners in August 2006, following their trade of Jamie Moyer to the Philadelphia Phillies.

Baek hit his first career home run on July 20, 2008, off Jaime Garcia.

Baek signed with the Chiba Lotte Marines of Nippon Professional Baseball for the 2015 season.