Mill Reef was bred in the United States of America 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 better suited him to a career on the turf courses in Europe rather than the dirt tracks in America and so he was sent to England in December 1969 to be trained by Paul Mellon's young English trainer Ian Balding at Kingsclere. He was 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.