Ahrens attended Memorial High School in Hedwig Village, Texas. At the 2006 World Wood Bat Championships, he led the Houston Heat with game-winning hits in the quarter- and semifinals. He was named player of the year in the Greater Houston area and was twice selected all-state and three times all-district. Prior to the 2007 draft, he signed to play baseball with Texas A&M University.Sports Illustrated projected Ahrens as a top-20 prospect for the 2007 draft.
Ahrens was drafted by the Toronto Blue Jays 16th overall in the draft. He decided to forgo college and enter the Blue Jays organization. He was given a $1.44 million US signing bonus, the 10th-highest for a Blue Jay prospect, which included $60,000 for schooling to offset his not going to Texas A&M University. He made his professional debut that season with the Gulf Coast Blue Jays. He played in the Blue Jays organization until 2013 without reaching the majors. Prior to the 2014 season he signed with the Atlanta Braves.