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.

1970: two-year-old season edit

Mill Reef made his debut in May in the Salisbury Stakes at Salisbury, where he beat the previous winner and 2-9 favourite, the Lester Piggott-ridden Fireside Chat, by four lengths.^[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.

Back on home soil, he was entered in the Gimcrack Stakes at York in mid August. After a torrential overnight downpour turned the going into a quagmire, his trainer wanted to scratch him from the race. However, after discussions with his owner prior to the race, Paul Mellon said, "Let him run, I've a feeling it will be alright." Mill Reef won by ten lengths from Green God (who was crowned champion sprinter the following year).^[4][5] In his next race, he beat the filly Hecla by a length in the Imperial Stakes at Kempton^[6] . In his final race of the season, he won the prestigious Dewhurst Stakes at Newmarket by four lengths.^[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.

1971: three-year-old season edit

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.

Although his breeding hinted otherwise, Mill Reef then proved himself to be the outstanding middle distance racehorse of the year, winning the Derby by two lengths from Linden Tree,^[11][12] the Eclipse Stakes at Sandown (beating the French colt Caro by four lengths)^[13][14] and the King George VI and Queen Elizabeth Stakes at Ascot by six lengths from Derby Italiano winner Ortis.^[15]

In October, he was victorious in the Prix de l'Arc de Triomphe at Longchamp in France, beating the French filly Pistol Packer by three lengths.

1972: four-year-old season edit

Kept in training as a four-year-old, Mill Reef returned to Longchamp to win the Prix Ganay in April 1972 by ten lengths. A summer rematch with Brigadier Gerard, who had also been kept in training as a four-year-old, was earmarked for the Eclipse Stakes at Sandown. 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.

After his recovery, Mill Reef was trained for an autumn campaign and a return to the Prix de l'Arc de Triomphe in October. During a routine training gallop, however, on 30 August he stumbled and shattered his foreleg. Charles Allen, a veterinary specialist, was flown in. Mill Reef's fracture was complicated. A triangularly shaped piece of bone about two and a half inches long was broken from the lower end of the cannon bone and was considerably displaced. The inner sesamoid bone was completely shattered and the rim of the top of the main pastern bone was damaged. It seemed likely that the inner sesamoid bone was the first to break, with the result on the next step that the fetlock was not braced and the foot and the pastern were pointing outwards. The sheer pressure of the horse's weight caused the crumbling of the rim of the pastern bone and the breaking of the cannon bone.

It was decided that an operation would be performed in a building in Ian Balding's yard. 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. The injuries on the sesamoid bone or the rim of the pastern were avoided. The operation was successful. Professor Edwin James Roberts performed the operation and Mill Reef's life was saved. His stable lad, John Hallum, played a major role in nursing him for three months, six weeks of which saw the horse in plaster. After the painstaking operation, Mill Reef's racing career was over and he became a stallion at The National Stud in Newmarket in 1973.

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]

• Oaksey, John (1974). Mill Reef. Michael Joseph.