1904: two-year-old season

Cicero was unbeaten in five races as a two-year-old.^[6] At the Craven meeting at Newmarket in April he won the Fitzwilliam Stakes, beating the King's, horse Rosemarket who started favourite. Cicero beat Rosemarket again in the Woodcote Stakes at Epsom in June, this time as the even money favourite. He was then sent to Royal Ascot where he won the Coventry Stakes at Royal Ascot beating Vedas by two lengths. He won the July Stakes at Newmarket at the unusual odds of 1/20 and then carried 135lbs to victory in the valuable National Breeder's Produce Stakes at Sandown in mid July. He sustained an injury in the Sandown race and missed the rest of the season,^[7] but nevertheless went into the winter break as favourite for the Derby.^[8]

1905: three-year-old season

Cicero had not been entered in the 2000 Guineas, which was won by Vedas and instead made his debut in the Newmarket Stakes. He won easily from Llangibby^[9] and was made favourite for the Derby.^[10] Vedas was ruled out of the Derby by injury and at Epsom Cicero was sent off at odds of 4/11, making him one of the shortest-priced favourites in the race's history. The second favourite was the French-trained Jardy, who was also undefeated, but reportedly suffering from a respiratory infection which had affected many of the horses in the stables of his owner, Edmond Blanc. Ridden by Danny Maher Cicero tracked the leaders in the early stages before making his challenge in the straight. Unable to find a run along the rails, Maher switched Cicero to the outside and took the lead only to be faced by the "strenuous challenges" of Jardy and Signorino. He ran on strongly under pressure to win after a "bitter struggle"^[11] by three quarters of a length from Jardy with Signorino a head further back in third.^[12] The winning time of 2:39.6 was a new race record.^[13]

Lord Rosebery celebrated the victory in extravagant style with a firework display at The Durdans, followed by a “lavish treat” for the four hundred inmates of the local workhouse and a garden party for three-thousand local "working men".^[14]

On his next start Cicero attempted to defend his unbeaten record in the Eclipse Stakes at Sandown for which he started 8/13 favourite. His main rival was expected to be Val d'Or, a stable companion of Jardy and the winner of the Poule d'Essai des Poulains. Cicero took up the lead in the straight and was immediately challenged by the French colt. Maher rode a vigorous finish, but Val d'Or, who was receiving three pounds from Cicero, pulled ahead in the closing stages to win by a half a length.^[15]

Cicero then developed leg problems and, although considered as a contender for the St Leger^[16] he did not appear on the racecourse for the rest of the year.

1906: four-year-old season

On his four-year-old debut on 20 April 1906, Cicero carried 140 lbs to beat Shilfa, his only opponent, in the one and a half mile Biennial Stakes at Newmarket, recording a time of 2:32.8.^[17] He was then aimed at the Ascot Gold Cup, for which he started second favourite, but after racing in second, he faded badly and finished unplaced behind Bachelor's Button and Pretty Polly.^[18]

On the advice of his trainer,^[19] Cicero was then retired.