the st. louis cardinals selected mejia in the fourth round, with the 150th overall selection, of the 2012 mlb draft. he signed with the cardinals, receiving a $250,000 signing bonus, and made his professional debut with the batavia muckdogs of the class a-short season new york–penn league. he tore his anterior cruciate ligament during a july 31 game with the muckdogs. mejia played for the peoria chiefs of the class a midwest league and the palm beach cardinals of the class a-advanced florida state league in 2013. he began the 2014 season with palm beach, and was promoted to the springfield cardinals of the class aa texas league in july. he began the 2015 season with springfield, and was promoted to the memphis redbirds of the class aaa pacific coast league during the season.