Castellano grew up in a racing family in Venezuela. His father and uncle were jockeys and his brother, Abel Jr., is based in South Florida. He began riding in 1996, moved to the U.S. in 1997 to pursue his career, and relocated to the higher-profile New York circuit in 2001.
Castellano won the Breeders' Cup Classic on Ghostzapper in 2004 and has won one Breeders' Cup race a year from 2012-2015. He picked up his first victory in a Triple Crown race on Bernardini in the 2006 Preakness. In 2010 at Saratoga, he won five consecutive races on the third day of the season and captured the meet's biggest prize with a victory in the Travers on Afleet Express. He led the Saratoga jockeys in purse money earned in 2011 with victories in five Grade 1 stakes: the Ruffian, the Personal Ensign, the Ballerina, the Travers and the Diana. He also won the Grade 1 Just a Game, Maker's Mark, Ashland and Prioress and finished the season with a career-best $15,677,027 in purse earnings.
After setting a new record for purse earnings in 2013 with $26,213,507 and leading all riders by victories with 362 and by stakes wins with 63, Castellano earned his first career Eclipse Award. He earned a repeat victory as the Eclipse Award winner as outstanding jockey in 2014, when he led all jockeys by purse earnings with $25,056,464 and recorded 315 wins. Castellano made it a three-peat in 2015 when he exceeded his own previous single-season earnings record as his mounts in 2015 earned $28,120,767. He was also leading rider by number of wins in 2015 with 344, with a 23% win rate from his 1,507 mounts.