He made his debut against the Baltimore Orioles pitching one scoreless inning, giving up a hit and one walk, with one strikeout.

[5] Marcum went 3–4 for the Blue Jays in 2006 in 21 games, including 14 starts, with an earned run average (ERA) of 5.06.

However, on September 19, days after an abbreviated start in which he left with elbow pain, the Jays released the information that Marcum would need Tommy John surgery and would miss the rest of the 2008 season and likely all of 2009.

He made two starts with the Advanced-A Dunedin Blue Jays in early July, and pitching successfully on his rehab assignment, he then started twice with the Double-A New Hampshire Fisher Cats, and once more with Triple-A Las Vegas 51s before being shut down for the season to prevent him from injury again.

[5] On December 6, at the 2010 MLB Winter Meetings, Marcum was traded to the Milwaukee Brewers in exchange for infield prospect Brett Lawrie.

[11] On July 4, 2011, Marcum hit his first Major League home run, a grand slam off Daniel Hudson of the Arizona Diamondbacks.

[12] In the 2011 NLDS against Arizona, Marcum gave up a grand slam to Paul Goldschmidt, which sealed a win for the Diamondbacks.

[18] Marcum finished his lone season in New York with a 1–10 record, 5.29 earned run average, and 60 strikeouts in 781⁄3 innings pitched.

[19] While pitching in extended spring training, on May 22, 2014, Marcum was shut down after problems with his injured shoulder occurred.

[26] Despite a 14-35 record, Marcum coached three Rangers to Great American Conference all-conference honors in his first and only season with the team.

Marcum was hired by Missouri Southern State University to be the pitching coach for the Lions on August 10, 2016.