Cossack performed impressively in trial gallops at Danebury and had built up a considerable reputation before appearing on a racecourse. His first public run in July 1846 at Newmarket when he started favourite for the July Stakes. He ran poorly, however and finished only third to the filly Miami, who went on to win the following year's Epsom Oaks. Cossack did not race again in 1846.
