The first call rider for Coolmore's Ballydoyle, Joseph O'Brien became the youngest rider to win a Breeders' Cup race when he piloted St Nicholas Abbey to victory in the 2011 Breeders' Cup Turf, his first start in the event. The son of prominent trainer Aidan O'Brien, Joseph won the race at 18 years and five months of age, surpassing Fernando Jara who won the Breeders' Cup Classic at 18 years and 10 months in 2006.
Mostly riding in his native Ireland, O'Brien won the 2010 Irish champion apprentice jockey title in 2010, rode his first Classic race winner in 2011 when he rode Roderic O'Connor to an Irish 2,000 Guineas victory, a feat he repeated in 2012 and 2013. In 2012, he was named the Irish Champion Jockey after riding 86 winners.
O'Brien teamed up with Camelot in 2012 to win two legs of the English Triple Crown in addition to the Irish Derby. With his father Aidan, the pair became the first father-son/trainer-jockey combination to win the Epsom Derby.
In October of 2013, O'Brien broke a 20-year-old record when he rode his 117th winner of the year to beat the previous mark set by Mick Kinane. He went on to ride 126 winners during the 2013 flat racing season in England and Ireland, ending his season with the mount on Declaration of War in his first Breeders' Cup Classic appearance.
At 5'11, O'Brien is one of the tallest jockeys in the world. He is limited to riding mounts at 126 pounds or more and plans to move to steeplechasing in the future when he is done riding on the flat.