The racehorse trainer Ian Balding, who has died aged 87, saddled 1,755 winners on the flat, of which 123 were in Group races of the highest class. Yet across his four-decade career there was one horse that stood out above all others – the great Mill Reef, who won the Derby and the Prix de l’Arc de Triomphe in 1971.

Like most trainers worth their salt, Balding was prepared to acknowledge his good fortune in coming across such an extraordinary horse. But even though Mill Reef was special from the outset, the colt still had to be expertly managed.

After winning the Greenham Stakes at Newbury in April 1971 and then the Derby at Epsom with ease that June, the three-year-old Mill Reef, ridden by Geoff Lewis, progressed to win the Eclipse Stakes and the King George VI and Queen Elizabeth Stakes in the following months, before going on to tackle the Arc at the conclusion of a very long season in October.

Balding, still a young trainer at 32, was worried that by the time his horse got to the big race in Paris he would be “over the top” – worn out from the year’s punishing schedule and unable to give of his best.

He later described the management of Mill Reef into that race as “perhaps my greatest test as a trainer” and admitted that “I had this awful feeling that I had done too much with him”. In the end Mill Reef, with Lewis once again in the saddle, won comfortably by three lengths in a new course record to become only the second horse, after Sea-Bird – and still only one of seven – to win the Derby and the Arc in the same year.

That victory proved to be Balding’s most satisfying moment, and was testament to his wonderful understanding of horses. He ended the 1971 season with 48 winners, making him the champion trainer on the flat, while Mill Reef moved on, as a four-year-old, to win two further big races the next year, the Prix Ganay and the Coronation Cup. However, just as Mill Reef was being readied for a defence of the Arc, catastrophe struck when he broke a leg in training and had to be retired to stud.

Balding never had a horse as good as Mill Reef again, but he steered plenty of others to success, including Mrs Penny, winner of the Prix de Diane in 1980, Glint of Gold, which came second in the Derby behind Shergar in 1981, Selkirk, Europe’s champion miler in 1991 and 1992, and the difficult-to-manage sprinter Lochsong, who won the Prix de l’Abbaye in 1993 and 1994.

Born in the US, in New Jersey, Ian was the son of Ellie (nee Hoagland), an American, and Gerald Balding, a British racehorse trainer. When Ian was seven, the family moved to the UK so that Gerald could train horses at a yard in Weyhill, Hampshire, bought for him by an American friend, the millionaire Jock Whitney, who later became the US ambassador to the UK.

Ian attended Marlborough college, Wiltshire, and Millfield school, Somerset, before going on to Christ’s College, Cambridge, where he failed a degree in rural estate management but won a rugby blue and made a name for himself as an amateur jumps jockey.

After their father died in 1957, Ian’s older brother, Toby, took over the Weyhill operation. When Ian finished at Cambridge he began to help Toby out, and in 1964 he accepted an offer to become an assistant to another trainer, Peter Hastings-Bass, at Kingsclere in Berkshire. Less than three months later Hastings-Bass died, at the age of 43, and Ian was invited by his widow, Priscilla, to take over his training licence.

Only 25 at the time, Balding was not even convinced that racehorse training was the right path to follow, and having won the National Hunt Chase as a jockey at the Cheltenham festival in 1963 he was still keen to keep up with his competitive riding. He was also unsure about going into opposition against his brother.

However, under the circumstances Balding accepted what was an exceptional, if daunting, opportunity. The yard had a string of good horses whose owners included Queen Elizabeth II, and within a short space of time he had sent out two winners at Royal Ascot and 10 winners in the royal colours. In his first complete year, 1965, he finished second in the trainers’ table with 43 victories.

Among the owners Balding inherited was the super-rich American Paul Mellon, and it was he who bred Mill Reef, delivering him to the yard in 1969.

When Lewis rode the two-year-old to a comfortable victory in his first race, at Salisbury in 1970, he told Balding he had got himself a once-in-a-lifetime horse. Neat and strongly made, acting on all types of ground and just as effective over a mile and a quarter as a mile and a half, he won 12 of his 14 races, finishing second in the other two, and after being sent to stud produced two Derby winners, Shirley Heights and Reference Point.

Though nothing could match the Mill Reef heights, the rest of Balding’s career gave him plenty of joy, and he even had 123 winners over the jumps, training Insular to the Imperial Cup at Sandown in 1986 and Crystal Spirit to the Sun Alliance Novices’ Hurdle at Cheltenham in 1991. After retiring in 2002 he handed over his Kingsclere yard to his son, Andrew.

In 1969 Ian married Emma Hastings-Bass, daughter of Priscilla and Peter. She survives him, as do their children, Andrew and Clare.

• Ian Anthony Balding, racehorse trainer, born 7 November 1938; died 2 January 2026