Among other unpleasant traits he kicked a spectator,[4] delayed the start of the race by repeatedly "rearing"[5] and attempted to attack and bite his own jockey, the highly experienced John Watts.

In the July Stakes at Newmarket the "ramapagious beast"[6] threw Watts and galloped free for several minutes prior to the start before finishing unplaced again.

[10] On his final start of the year two weeks later, he finished second behind Democrat again in the Dewhurst Stakes, although he produced an improved effort and gave the winner a more difficult race on this occasion.

[11] Despite his relatively modest form Diamond Jubilee was seen as a colt with Classic potential,[12] partly because his pedigree suggested that he would improve with age and distance, whereas Democrat was regarded as essentially a precocious, sprinting type.

Two furlongs from the finish, Jones sent Diamond Jubilee into the lead and he stayed on strongly to win by half a length from Simon Dale, with Disguise third.

At Newmarket the colt started 4/5 favourite for the £10,000 Princess of Wales's Stakes over one mile on 5 July, for which as the Derby winner, he had to carry a weight penalty.

He was unable to give twenty pounds to the filly Merry Gal, who led from the start and won by four lengths,[20] but finished ahead of some good horses and was felt to have run creditably.

He produced his most impressive performance to date,[22] taking the lead in the straight and winning decisively from Chevening, to whom he was conceding ten pounds.

[24] On 12 September, Diamond Jubilee attempted to become the ninth horse to complete the Triple Crown[25] in the St Leger at Doncaster.

[27] On this occasion however, he was unable to reproduce his best form, and after racing prominently for part of the way, he dropped out in the closing stages and finished unplaced behind Disguise.

[30] Diamond Jubilee's earnings of £27,985 were largely responsible for ensuring that the Prince of Wales was the leading owner, in terms of money won for the 1900 British season.

[35] In the Princess of Wales's Stakes on 4 July he finished second to Epsom Lad, who was winning his first race since beating Diamond Jubilee as a two-year-old.

[37] Diamond Jubilee ran his last race on 3 October at Newmarket when he finished third to Pietermaritzburg and Epsom Lad in the Jockey Club Stakes,[38] after which he was retired to stud.