Bred by the Hon [Robert] Edward Petre (1794-1898), Theodore was a half-brother to to the Champion Sire Blacklock (b c 1814 Whitelock). Petre won the St Leger four times, with Theodore, Matilda (b f 1824 Comus), The Colonel (ch c 1825 Whisker) and Rowton (ch c 1826 Oiseau).

His turf career spanned five years during which he started twenty-one times, was victorious in nine of those, finished second seven times, third once, and went unplaced four times.

His victory in the Great St Leger came as a surprise due to his unsoundness, severe enough to send him off at 100 to 1. Bets against him were said to include £1000 to a crown, and £100 to a walking-cane. Even his owner had so little hope that he sold his book to Rhodes Mills for £200 (who made thousands), and his jockey John Jackson reputedly burst into tears when told to ride what he termed "such a cripple as that". Nevertheless, Jackson followed his instructions to race at his best pace, went off on the lead, was never headed and won by four lengths.

He was purchased by William Carleton at the end of his four year old season and sent to France in 1838.