St Louis was a "grand"[3] bay horse with a white blaze bred at the Confey Stud in County Dublin by J J Maher.

St Louis's dam Princess Sterling was an influential broodmare whose female-line descendants included Noblesse who was in turn the ancestor of Rainbow Quest, Warning and Commander in Chief.

[7] Ridden by the American jockey George Archibald, St Louis stated at odds of 6/1 in a 22-runner field for the 114th running of the 2000 Guineas over the Rowley Mile at Newmarket Racecourse on 26 April.

[12] On 29 May St Louis started favourite for the Derby over one and a half miles put appeared to be unsuited by the firmer ground[5] and finished fourth behind Captain Cuttle, Tamar and Craigangower.

[13] St Louis was off the course until August, when he started at odds of 1/9 against four moderate opponents in the Wolverhampton Breeder's Foal Plate over ten furlongs.