1810: three-year-old season

Hephestion began his racing career on 23 April the opening day of the 1810 flat racing season at Newmarket Racecourse. He started at odd of 5/1 for a produce sweepstakes over the Rowley Mile course and finished second of the six runners behind Pledge, a filly owned by the Duke of Grafton.^[5] At the next Newmarket meeting, Hephestion was one of nine three-year-olds, from an original entry of twenty-seven, to contest the second running of the 2,000 Guineas Stakes over the Rowley Mile. Ridden by Frank Buckle, he was made the 5/1 second favourite and won easily^[6] from Lord Kinnaird's colt The Dandy.^[7]

A month after his win at Newmarket, Hephestion was moved up in distance to contest the Derby Stakes over one and a half miles at Epsom Downs Racecourse. He started the 3/1 second favourite but finished unplaced behind Whalebone and The Dandy.^[8]

1811: four-year-old season

Hephestion won four of his ten competitive races as a four-year-old in 1811. After being off the course for more than ten months, he reappeared at Newmarket in May in the colours of Mr Ladbroke. He won a sweepstakes over ten furlongs at the first spring meeting^[9] but at the next meeting two weeks later was beaten by Asmodeus in a five furlong sweepstakes and by Lord Oxford's filly Morgiana in a handicap race. By the end of that month he had entered the ownership of Sir H. Lippincott and at the Epsom Derby meeting he finished third in the Gold Cup over two miles^[10] after which he was campaigned at relatively minor courses. At Stockbridge Racecourse a week later he finished last of three runners in a sweepstakes.^[11] At the end of June Hephestion had three engagements at the Bibury Club meeting at Burford: he was withdrawn from the Sherborne Stakes, finished third in a four mile sweepstakes, received compromise when his rival failed to appear for a match race and then recorded his first competitive win of the season when he won a one mile match against a six-year-old mare named Matilda.^[12] His next appearance was at Bath Racecourse on 5 July, when he contested a handicap race run in a series of one mile heats, with the prize going to the first horse to win twice. Hephestion won the first heat, finished second in the next two, and claimed the victory when his only remaining rival, a mare named Viscountess, fell in the deciding heat.^[13] Two weeks later, he finished third behind Romana in the Cup over one and a half miles at Winchester Racecourse. On 30 July Hephestion ended his summer campaign by winning both heats of a £50 race at Stockbridge from two opponents. ^[14] His final appearance of the season was on 17 September at Kingscote Racecourse, where he finished third in the Kingscote Stakes over three miles.

1812: five-year-old season

Hephestion remained in training as a five-year-old but failed to win in four races. At the Maddington Club meeting at Stockbridge in May he was beaten by Ringdove in a 50 guinea match race and then finished third of the four runners behind Romeo in the four-mile Maddington Stakes.^[15] In the following month he finished last of three behind Ringdove in a claiming race at the Bibury meeting. Hephestion ended his racing career at Kingscote in September, winning the first heat of a two-mile handicap but finishing second in the next two heats behind Topsy-Turvy.^[16]

Hephestion disappears from the records after his defeat at Kingscote. He does not appear on any lists of stallions and has no offspring mentioned in the General Stud Book.