[2] William the Third was one of the last notable horses sired by St. Simon, an undefeated racehorse who was considered one of the best British runners of the 19th Century.

His dam, Gravity, was a daughter of the influential broodmare Enigma, the direct female ancestor of many major winners including Reigh Count, Candy Spots and Martial.

[2] Later that month, the colt established himself as a classic contender when he won the ten furlong Newmarket Stakes by a short head, beating Doricles, a horse who finished second in the 2000 Guineas on his previous start.

[5] William the Third was then sent to Epsom Downs Racecourse to contest the 122nd running of the Derby Stakes for which he started at odds of 100/7 in a field of twenty-five runners.

On his debut he was sent to Royal Ascot for the two-and-a-half-mile Gold Cup which attracted an extremely strong field.

The runners included the classic winners Volodyovski and Cap and Bells (Epsom Oaks), the leading British older horses Osbech (Coronation Cup) and Santoi (1901 Ascot Gold Cup), and the French challengers Cheri (Grand Prix de Paris) and La Camargo (Prix de Diane).

[7] The day attracted a large and fashionable crowd to Ascot, including the Queen and the Prince and Princess of Wales Ridden by Morny Cannon, William the Third, who started the 2/1 favourite, was held up towards the back of the field of runners until the straight.

In their book A Century of Champions, based on an adaptation of the Timeform system, John Randall and Tony Morris awarded William the Third a rating of 135, making him a "superior" Gold Cup winner, and the best British racehorse of his generation.