He was the ninth of thirteen foals produced by Mr Garforth's unnamed Comus mare, whose other progeny included the Northumberland Plate winner Hetman Platoff.

[3] Until 1913, there was no requirement for British racehorses to have official names,[4] and the horse who later became known as Don John began his racing career as Lord Chesterfield's b. c. by Tramp, or Waverley, out of Sharpset's dam.

A field of seven runners, the smallest for thirty-six years, assembled for the Great St Leger Stakes and Don John was made 13/8 favourite ahead of the Derby runner-up Ion.

Ridden as usual by Bill Scott, Don John took the lead shortly after the start, quickly took a three length advantage and set a "killing" pace.

[12] His victory was enthusiastically received the large crowd, and became even more popular when Lord Chesterfield held "open house" at the nearby Salutation Inn, providing free food and drink to anyone who joined the celebration.

[13] Two days later the St Leger winner was matched against older horse in the four-mile Gold Cup, a race in which he was partnered by Flatman as Scott was unable to ride at the specified weight of seven stones.

[14] Don John tracked Bee's-wing until the final furlong when he took the lead, quickly went clear and won easily from the mare, with The Doctor third and Melbourne last of the four runners.

He started 2/5 favourite for the race but was beaten by Lord George Bentinck's colt Grey Momus winner of the previous seasons 2000 Guineas and Ascot Gold Cup.

[17] At Newmarket's First Spring meeting later that month Don John was ridden by Harry Edwards to win an 1,800 sovereign sweepstakes from three opponents over the four-mile Beacon Course.

[24] In November 1855, Don John won second prize at the Virginia State Fair in the Thoroughbred stallion category, losing to Red Eye by Boston.

As the founder of Family 9-e she was the direct female-line ancestor of numerous major winners throughout the nineteenth and twentieth centuries: her descendants include Cyllene, Friar Rock, Corrida, Galcador, Haafhd, Snow Chief, High-Rise, Zafonic, Daylami, Dalakhani and Dubawi.