On August 20, 2009, Mejía signed with the Detroit Tigers as an undrafted free agent after he was not selected in the 2009 Major League Baseball draft.

Mejía subsequently signed with the Bridgeport Bluefish of the Atlantic League of Professional Baseball.

[4] In 15 games split between the Low–A Jamestown Jammers and Single–A Greensboro Grasshoppers, he registered a 3–3 record and 3.18 ERA with 26 strikeouts over 22+2⁄3 innings pitched.

[6] In 21 games (18 starts) for the Elephants, Mejía compiled a 5–9 record and 3.26 ERA with 77 strikeouts across 107+2⁄3 innings pitched.

[7] In 55 appearances out of the bullpen for the Monkeys, Mejía compiled a 5–1 record and 1.24 ERA with 46 strikeouts across 58 innings pitched.

He made 25 appearances split between the Double–A Tennessee Smokies and Triple–A Iowa Cubs, accumulating a 1.38 ERA with 18 strikeouts across 26 innings pitched.