[2][3][4] After his junior year, Jackson was selected by the Toronto Blue Jays in the first round (32nd overall) of the 2004 Major League Baseball draft.

[7] On December 7, 2005, Jackson was traded along with Dave Bush and Gabe Gross to the Milwaukee Brewers for Lyle Overbay and Ty Taubenheim.

[10] He made his major league debut on June 4 against the Washington Nationals, tossing a perfect ninth inning while striking out two in an 8–4 loss.

[13][16] On July 7, 2008, Jackson was part of the deal that sent Cy Young award winner CC Sabathia to Milwaukee for left fielder Matt LaPorta, pitcher Rob Bryson, and Michael Brantley.

[17][18] Jackson was recalled in August 2008 to fill the starting pitcher spot vacated by Paul Byrd, who had been traded to the Boston Red Sox.

[13] Jackson spent the majority of the 2009 season with the Triple-A Columbus Clippers, posting a 4–8 record with a 6.05 ERA in 30 games (14 starts).

[13] He was converted to a reliever in the Royals system, and was 2–1 with 18 saves and a 1.38 ERA in 33 relief appearances with the Double-A Northwest Arkansas Naturals.