On July 30, 2007, Harrison, along with catcher Jarrod Saltalamacchia, minor league pitchers Neftalí Feliz and Beau Jones, and shortstop prospect Elvis Andrus, were traded by the Braves to the Texas Rangers for Mark Teixeira and Ron Mahay.

[3] Harrison missed the end of the 2007 minor league regular season due to shoulder inflammation.

[4] He made his first appearance for the Rangers organization with the Surprise Rafters during the 2007 Arizona Fall League season, where he accounted for a perfect 5–0 record.

As the middle of the 2008 season wore on, the Rangers pitching staff suffered a number of injuries, and the team made the decision to call up Harrison from Frisco on July 8.

[10] With the Rangers finishing 90–72, the team made the postseason but lost to the San Francisco Giants in the 2010 World Series.

[10] He started the game the Rangers won to clinch the AL West (Pitched 6 innings, gave up a run).

[citation needed] Harrison won Game 4 Of the ALDS versus the Tampa Bay Rays.

On May 13, 2014, Harrison left the game with an apparent back injury, which landed him on the disabled list the following day.

On June 3, 2014, Harrison underwent spinal fusion surgery to alleviate lumbar herniation in his back, which ended his season.

Doctors declared that it was not safe for Harrison to continue playing baseball or any other sport, even after making a full recovery.

Harrison started the 2015 season on the 60-day disabled list in an effort to continue recovery from his spinal fusion surgery.