Clayton Grant Mortensen
- Bats Right, Throws Right
- Height 6' 4", Weight 180 lb.
- School Treasure Valley Community College, Gonzaga University
- High School Madison High School (Rexburg)
- Debut June 29, 2009
On July 24, 2009, less than a month after his major league debut, Mortensen was traded by the Cardinals with Shane Peterson and Brett Wallace to the Oakland Athletics in return for Matt Holliday. He made 6 starts for the Athletics late in 2009, going 2-4, 7.81. He then made only one appearance in 2010, a start in which he pitched 6 innings, giving up 3 runs with a no-decision. He was traded to the Colorado Rockies for Ethan Hollingsworth before the 2011 season and managed to stick in the big leagues for a while that year, making 16 appearances including 6 starts and pitching 58 1/3 innings. His record was only 2-4, but with a solid 3.86 ERA.
Mortensen was traded to the Boston Red Sox in return for IF Marco Scutaro on January 21, 2012. He was a full-time reliever that season, pitching 26 times and logging 42 innings with a 1-1 record and a 3.21 ERA. He kept his job in the Red Sox's bullpen at the start of 2013, but did not pitch as well, as his ERA was 5.34 after 24 games. On June 29th, he was designated for assignment in a roster crunch, as the Sox needed a roster spot for an extra infielder - Jonathan Diaz - with SS Stephen Drew unavailable for a few days but not on the disabled list. He went back to the minor leagues, then on August 27th was traded to the Kansas City Royals in return for OF Quintin Berry.