 In 2005, Thornton served his reliever duties, pitching in 55 games with 57 innings pitched and posting a 5.21 ERA and striking out 57.On March 21, , Thornton was traded from the Seattle Mariners to the Chicago White Sox in exchange for outfielder Joe Borchard.Under the tutelage of pitching coach Don Cooper, Thornton emerged as a dominant bullpen force.