Mercedes was signed as an undrafted free agent by the Tigers in 2008 out of the Dominican Republic. After beginning his career in the Dominican Summer League, he quickly ascended the Tigers minor league ranks. He made his Major League debut for Detroit at age 23, working two perfect innings in relief in a game against the Mariners. He rung up Kyle Seager for his first Major League punch out, and came back to strike out the next batter, Mike Zunino, as well.

Last season Mercedes pitched in the Mexican League for Oaxaca, where he posted a 5-2 record and a 4.73 ERA in 35 appearances. In his minor league career he has won 16 games to go along with a 3.56 ERA.