Though he was born in Puerto Rico, Mercedes grew up in La Romana, Dominican Republic. For his tryouts with Major League clubs back home in the Dominican Republic he assumed the name of his cousin, Domingo Cuello so that scouts would think he was four years younger. On November 5, 1999 he signed for $22,000 with the Pittsburgh Pirates and, because they thought he was still 17, played a season of developmental ball before he debuted with the Williamsport Crosscutters of the New York-Penn League in 2001.
In 2002 Mercedes' false identity was discovered by the United States Consulate in the Dominican Republic. Because he was actually born in the United States, though, he was able to travel without a visa to return for the 2002 baseball season. He remained with Pittsburgh, playing for the Hickory Crawdads and the Lynchburg Hillcats in single-A ball. The Washington Nationals picked him up as a free agent in 2005, sending him to the Potomic Nationals and the AA Harrisburg Senators for the season. The Chicago White Sox signed him as a free agent on April 4, 2006.
A switch hitter, he batted .276 for the second half of the 2007 season after he was promoted from the Class A Winston-Salem Warthogs. He delivered several memorable moments including hitting three doubles against the West Tennessee Diamond Jaxx on July 25 and a five-RBI night against the Mobile BayBears on August 7. During the June 10 game against the Mississippi Braves he became the first Baron to hit for the cycle since Reggie Jackson accomplished it on July 8, 1967.
In 2009 Mercedes returned to the Southern League with the Chattanooga Lookouts. In 2010 he played for the Guerreros de Oaxaca in the AAA Mexican League. He remained in Mexico in 2011 with the Vaqueros Laguna and the Olmecas de Tabasco. The next season he started with Oaxaca, but then returned to the Dominican League, playing six games with the Gigantes del Cibao before retiring from baseball.
- Segrest, Doug (June 30, 2008) "A.K.A. talented." The Birmingham News