He won both his starts as a four-year-old before his racing career was ended by a severely broken leg that threatened his life.
Mill Reef was bred in the United States at the Rokeby Stables in Virginia of his owner and breeder, philanthropist Paul Mellon.
When he was a yearling, it was thought that his action would be a better fit for a career on the turf courses of Europe rather than the dirt tracks in America.
Once, whilst visiting the stables and watching the yearlings being put through their paces on the Kingsclere gallops, the noted former amateur jockey and journalist Lord Oaksey asked, "Who's that?"
[2] He then went to Royal Ascot winning the Coventry Stakes by six lengths,[3] and the decision was made to go to France for the Prix Robert Papin at Maisons-Laffitte.
After an arduous journey, Mill Reef tasted defeat for the first time by the narrowest of margins to another exceptional English two-year-old, My Swallow.
[7][8] In a crop of outstanding two-year-olds, Mill Reef was rated 1 lb below his French conqueror My Swallow, who remained unbeaten in seven races including all of France's top two-year-old races, and 1 lb ahead of the unbeaten Middle Park Stakes winner, Brigadier Gerard.
As a three-year-old, following a victory in the Greenham Stakes at Newbury,[9] Mill Reef was beaten three lengths in the 2,000 Guineas by Brigadier Gerard,[10] who was to prove himself one of the best milers ever, with his old rival My Swallow back in third.
Kept in training as a four-year-old, Mill Reef returned to Longchamp to win the Prix Ganay in April 1972 by ten lengths.
But after a narrow win in the Coronation Cup at Epsom Downs, by a neck from Homeric,[16][17] Mill Reef was found to be suffering from a heavy virus and the rematch had to be postponed.
Over a span of six hours, a simplified stainless-steel compression plate held by three screws was used to pin the broken pieces to the cannon bone.
His stable lad, John Hallum, played a major role in nursing him for three months, six weeks of which saw the horse in plaster.