[1] He made his professional debut in the Gulf Coast League (GCL), going 0–2 with an earned run average of 5.40 in seven games.

Mathieson made his MLB debut June 17, 2006, losing to the Tampa Bay Devil Rays, 7–2.

Mathieson began a rehabilitation program late in the 2007 season, but removed himself from a game August 31 while pitching for Double-A Reading when he felt "a strange sensation in his elbow.

[5] On June 17, 2010, Mathieson was recalled from the Lehigh Valley IronPigs to the Philadelphia Phillies,[6] where he remained for several weeks.

[8] With the conclusion of the 2016 NPB season, his fifth year in Japan with the Yomiuri Giants, Mathieson had appeared in 300 games as a relief pitcher, compiling a 21–20 record with 2.32 ERA and 43 saves.