 Robertson was drafted in the 31st round of the 2004 MLB Draft by the Oakland Athletics out of Birmingham–Southern College.He spent four years working his way up the Oakland minor league system before making his major league debut on May 17 in which he pitched an inning and two thirds in an Oakland 4-7 loss to the Royals; Robertson made two more appearances in the 2007 season and was optioned back to AAA where he pitched as the closer before breaking his thumb and missing the rest of the 2007 season.On December 14, 2007 Robertson was traded with starting pitcher Dan Haren to the Arizona Diamondbacks for left-handed pitchers Brett Anderson, Dana Eveland and Greg Smith, infielder Chris Carter and outfielders Aaron Cunningham and Carlos Gonzalez.