He won several minor races as a three-year-old in 2009 but then lost his form and failed to win in the next two seasons, including and unsuccessful stint as a National Hunt horse.
He finally emerged as a top-class performer at the age of six when he rebounded from an unlucky defeat in the Ebor Handicap to record an upset victory in the Irish St. Leger.
His other progeny included Workforce, Creachadoir, Proclamation, Eishin Flash (Tokyo Yushun, Tenno Sho), Sajjhaa (Dubai Duty Free Stakes) and King's Apostle (Prix Maurice de Gheest).
On 29 July at Sandown Park Racecourse Royal Diamond was assigned a weight of 123 pounds in a handicap and recorded his first success as he won by two and a half lengths at odds of 6/1.
[7] The gelding remained unbeaten for the rest of the 2009 season, although his campaign was restricted to minor handicaps over extended distances: he won at Yarmouth in August, Ffos Las in September and Wolverhampton in October.
For the 2012 Flat season, Royal Diamond was moved to Ireland and joined the stable of the former steeplechase jockey Tommy Carmody at the Curragh in County Kildare.
After finishing third in a minor race at Dundalk he recorded his first win in two and a half years when he was ridden to victory by Johnny Murtagh in a handicap at Leopardstown on 15 April.
Fame and Glory set a steady pace with Royal Diamond, partnered by Niall McCullagh, settled in fourth place in the early stages before moving into contention approaching the final turn.
Fame and Glory went clear of the field in the straight but then began to struggle as Massiyn, Brown Panther, Aiken and Royal Diamond all stayed on strongly.
A week after his run at York, Royal Diamond changed trainers yet again, moving to the recently established stable of Johnny Murtagh at Coolaghknock Glebe, County Kildare.
Royal Diamond was beaten five lengths into second by the Aidan O'Brien-trained Ernest Hemingway in the Curragh Cup in June and was then dropped in class for the Listed Challenge Stakes at Leopardstown Racecourse in July.
In the Irish St. Leger Trial Stakes sixteen days later he faced Ernest Hemingway yet again in a five-runner field which also included Sir Ector and the improving four-year-old filly Voleuse de Coeurs.