Mattheus was selected by the Colorado Rockies in the 19th round of the 2003 Major League Baseball draft.

He pitched a complete-game shutout on May 28 against the San Jose Giants, retiring 23 of 32 batters faced in the Nuts' 4–0 victory.

[1] Mattheus bounced around the Rockies' organization in 2009, playing for Modesto again, Tulsa again, and finally made his first appearance at the Triple-A level with the Colorado Springs Sky Sox.

[2] On July 31, 2009, Mattheus was traded with Robinson Fabian for major league relief pitcher Joe Beimel.

He retired the next two batters, and got all three outs on just two pitches without allowing a run (by getting a groundout and then a ground-ball double play), a first in postseason history.

[8] Mattheus started the season on the Opening Day roster for the Salt Lake City Bees, the Angels' Triple–A affiliate,[9] and was called up to the majors on May 8.

With the Cubs leading 4–3 in the bottom of the fifth inning, Mattheus relieved starter Michael Lorenzen, who had left runners on first and second with two outs.

Kris Bryant hit Mattheus' only pitch for an RBI single, but after the run, which made the score 5–3 in the Cubs' favor, Anthony Rizzo was thrown out trying to take third.

Mattheus signed with the Sugar Land Skeeters of the Atlantic League and posted a 3.00 ERA in 38 relief appearances.

Mattheus has four pitches in his repertoire: a sinker and four-seam fastball averaging about 94 mph, and a slider and splitter with good sinking action in the mid 80s.