It didn't take long for Rosario to become one of the top riders in California after arriving from the Dominican Republic in 2006. He grew up on a farm outside of Santo Domingo and decided to be a jockey after attending the races when he was 13. After six months in jockey school, he turned pro at the age of 14. Rosario was the leading rider in the Dominican Republic four times before deciding to test his skills in California. He came to the U.S. in 2006 and rode in Northern California for much of 2007. Rosaria moved to Southern California late in 2007 and by early 2008 was riding stakes horses for major trainers. He moved into the upper echelon in 2009 when he finished fifth in earnings with $13 million. Rosario climbed to third place in 2010. He won his third-consecutive Del Mar and Hollywood Park titles and his first Santa Anita crown in 2011.
In 2012, Rosario moved to the East Coast and quickly established himself as one of the top riders on the other side of the country.
Rosario started his 2013 season with a huge win in the $10 Million Dubai World Cup aboard Team Valor's Animal Kingdom. Shortly after, the Dominican Rupublic native picked up a mount for the 2013 Kentucky Derby with Shug McGaughy's trainee Orb, and he guided the Malibu Moon colt to a dominant win in the first jewel of the Triple Crown to earn his first win in the race. Rosario was aboard Orb for a fourth place finish in the Preakness, and kept the mount for the Belmont Stakes where they finished third.
Rosario had another very good year in 2014, winning the Belmont Stakes on Tonalist. He also had the regular mount on 2014 champion older mare Close Hatches, winning three grade one races on her that year. He ended his year with a win in the 2014 Breeders' Cup Turf Sprint on Bobby's Kitten for Ken and Sarah Ramsey and trainer Chad Brown. Overall, Rosario won 11 Grade 1 races in 2014.