A left-handed starting pitcher, Mulder pitched in Major League Baseball (MLB) for the Oakland Athletics and St. Louis Cardinals.

Leading the American League with 21 wins, he anchored a powerful Oakland rotation along with Barry Zito and Tim Hudson, called "The Big Three".

[citation needed] Mulder finished second in AL Cy Young Award voting behind Roger Clemens.

He carried over his strong regular-season performance by pitching 24 innings in the four playoff starts, with an ERA of 2.25 and 19 strikeouts.

After the 2004 season, the Athletics traded Mulder to the St. Louis Cardinals in exchange for Dan Haren, Kiko Calero, and Daric Barton.

He turned out to be suffering from rotator cuff and shoulder problems, and the Cardinals placed him on the disabled list on June 23.

After undergoing rotator cuff surgery, and with a return for the opening of the 2007 season unlikely, Mulder's future with the Cardinals looked somewhat uncertain in the 2007 offseason.

This prompted an MRI scan, which led the team to the conclusion that Mulder needed additional clean-up rotator cuff surgery.

Although he was expected to recover from surgery in time for Spring training, Mark started the 2008 season on the disabled list.

[9] Mulder expressed going into spring training that his shoulder felt strong, but wondered about the susceptibility of his lower back and legs to injury.

He suffered the injury in agility drills on the second day of spring training, before he was scheduled to throw his first bullpen session.