Mill Reef (23 February 1968 – 2 February 1986) was an American-bred, British-trained Thoroughbred racehorse and sire. In a three-year career from 1970 to 1972, he won twelve of fourteen races and finished second in the other two. The horse was an outstanding two-year-old in 1970 but proved even better at three, winning the Epsom Derby, the Eclipse Stakes, the King George VI and Queen Elizabeth Stakes and the Prix de l'Arc de Triomphe. 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. At not more than 15.4 hands as a three-year-old,^[1] Mill Reef was quite a small horse but is considered one of the greatest thoroughbreds to run in Europe since WWII.

He recovered from a complicated operation on his broken leg to become a very successful stallion.

Mill Reef was bred in the United States at the Rokeby Stables in Virginia of his owner and breeder the philanthropist Paul Mellon. He was a son of Never Bend out of the mare Milan Mill by Princequillo. As 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. Therefore, he was sent to England in December 1969 to be trained by Paul Mellon's young English trainer Ian Balding at Kingsclere. He would be ridden by Geoff Lewis in all his fourteen races. Mellon named the horse after the Mill Reef Club, which is situated on the island of Antigua in the West Indies. The Mellon family has maintained a home at Mill Reef since its founding in 1947.

As a yearling Mill Reef showed himself to be an exceptional talent. 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?" to which Balding replied, "That is Mill Reef!" He went on to prove himself an outstanding two-year-old in 1970.

Mill Reef's descendants include:^[18]

c = colt, f = filly

Mill Reef was euthanised on 2 February 1986 and he is buried within the National Stud, where a statue stands in his memory.

Mill Reef was given a rating of 141 by Timeform and is listed at no. 4 in the greatest racehorses of the twentieth century.

The Mill Reef Stakes at Newbury is named in his honour.

The following is inscribed on the plinth beneath his statue at the National Stud:

Swift as a bird I flew down many a course.
Princes, Lords, Commoners all sang my praise.
In victory or defeat I played my part.
Remember me, all men who love the Horse,
If hearts and spirits flag in after days;
Though small, I gave my all. I gave my heart.
(from Paul Mellon's speech at the Gimcrack Dinner 1970).

The Royal Veterinary College's Hawkshead Campus in Hertfordshire has a dedicated pathology building in memory of Mill Reef named the "Mill Reef Building".^[19]

Indian leg-spinner B. S. Chandrasekhar’s quicker ball was called the Mill Reef, a testimony to its pace. He famously used it to bowl John Edrich in the Oval Test match of 1971. ^[20]