Marcum was born in Kansas City, Missouri and raised in Excelsior Springs, Missouri, where he attended Excelsior Springs High School. Marcum initially attended the University of Missouri, but transferred to Missouri State University, playing on the 2003 College World Series team.
Marcum was drafted by the Blue Jays in the third round, 80th overall, of the 2003 Major League Baseball Draft. Marcum quickly rose through the minor leagues and made his major league debut on September 6, 2005 as a late season call-up from the minors in September when rosters expanded. He made his debut against the Baltimore Orioles pitching one scoreless inning, giving up a hit and one walk and one strikeout. Marcum pitched 8 innings, giving up six hits, without surrendering a run during the month of September.
Marcum went 3–4 for the Blue Jays in 2006 in 21 games, including 14 starts, with an ERA of 5.06. In his final seven starts, he had a record of 2–1 with a 3.31 ERA.
Marcum had a breakthrough season in 2007. He went 12–6 with an ERA of 4.13. He struck out a total of 122 batters over 159 innings of work. He pitched 6+ scoreless innings in seven of his starts, including two instances where he pitched 6+ no-hit innings before being relieved. One of those instances was against the Boston Red Sox.
In 2008, Marcum got off to a good start, going 5–4 with a 2.65 ERA and 86 strikeouts in 98.2 Innings. But an injury followed by a handful of weak starts sent him to Triple-A on August 23. In September he was back in the starting rotation and he seemed to have returned to form. 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.
As of May 1, 2009, Marcum had started to throw again. Marcum made two starts with A-Class Dunedin in early July, pitching successfully on his rehab assignment, he then started twice with the AA-class New Hampshire Fisher Cats, and once more with AAA Las Vegas 51s before being shut down for the season to prevent him from injury again.
On March 22 Marcum was named the opening day starter for the 2010 season, succeeding Roy Halladay for that role, who had seven consecutive opening day starts for the team from 2003 to 2009. On May 2, Marcum got his first win against the Oakland Athletics, it was his first win since September 11 2008 before going through Tommy John surgery. On August 4, 2010, Marcum gave up Alex Rodriguez's 600th career home run. Marcum ended the season with a 13-8 win–loss record and a 3.64 ERA.
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