He was selected by the Blue Jays out of UCLA in the fourth round of the 2004 draft. When an injury to A.J. Burnett created an opening in the Blue Jays' rotation, Janssen was promoted from the Syracuse Chiefs and made his major-league debut against the Baltimore Orioles on April 27, 2006.
Janssen throws six different pitches: a four-seam fastball that tops out at 92 mph, a two-seamer, a cutter, a curveball, a change-up and a slider. Although he is not an overpowering pitcher, thus far in his career Janssen has been very successful in upsetting hitters' timing and inducing weakly hit balls. In his first two months in the majors, he proved to be a groundball pitcher who allows few line drives.
Casey Janssen won his first two games against the Los Angeles Angels, both times posting extremely solid outings, allowing less than three hits over seven innings each.
In 2007, with multiple injuries to the Blue Jays rotation and bullpen, Janssen made a move to the bullpen and performed very well as the set-up man for interim closer Jeremy Accardo. He led the bullpen in innings pitched, and was second in team saves with six. Janssen established himself as one of the best in the Toronto Blue Jays bullpen as well as in the American League.
Janssen missed the entire 2008 season with a torn labrum. He was expected to make a full recovery and to be ready for Spring Training 2009, whether it be out of the bullpen or as a starter. But he suffered another injury setback, and did not make the Opening Day 2009 roster for Toronto.
On Saturday May 23, Janssen returned to MLB action facing the Atlanta Braves in inter league play going 6 innings and giving up 8 hits and 3 earned runs in a 4-3 decision giving him his first loss of the season.
On June 17, 2009, it was announced that Janssen was placed on the 15-Day disabled list, retroactive to June 14, with inflammation of his right shoulder.