1846: two-year-old season 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.
1847: three-year-old season At
Newmarket's spring meeting, Cossack started
6/5 favourite and won the
Newmarket Stakes, beating War Eagle by a length, with the pair pulling well clear and leaving the other runners in "a cloud of dust". Ridden by Sim Templeman, Cossack was second in the early stages, but moved into the lead by half way and soon opened up a clear advantage and was being cheered by the crowd as the likely winner half a mile from the finish. Cossack won easily by a length from War Eagle, with Van Tromp, who had been badly hampered early in the race, At
Royal Ascot in June, Cossack was allowed to
walk over for the Swinley Stakes, when the other runners where withdrawn. At
Doncaster in September, Cossack ran in the
St Leger. He started favourite at odds of 4/5 despite doubts about his stamina and the fact that Pedley "declared to win" with his other runner, Foreclosure. Templeman attempted to repeat his Epsom tactics and sent Cossack into the lead from the start. By the straight most of his rivals were struggling but Van Tromp, ridden by Job Marson made steady progress and moved alongside the favourite a furlong and a half from the finish. Marson sent his colt past the leader and Templeman, realising he was beaten, eased Cossack down in the closing stages to finish second, two lengths behind the winner but well clear of the rest. On 12 October, Cossack appeared at Newmarket for the two and a quarter-mile
Cesarewitch Handicap for which he was set a weight of 118
pounds, including a six-pound weight penalty for finishing second in the St Leger. He finished fourth, behind the four-year-old Cawroush, who was carrying 98 pounds. His performance was creditable and reportedly led to speculation that he could have won the St Leger, had he not been ridden so aggressively. Cossack's winnings for the season totaled £5,950, making him the second highest earner of the British season behind the
Duke of Richmond's Red Hart.
1848: four-year-old season Cossack ran twice without success at
Goodwood in July 1848. On the Tuesday of the meeting he faced Van Tromp in a £300
Sweepstakes over three and a half miles. Van Tromp led from the start and Cossack, who was eased down when his chance had gone, was beaten by an estimated 150 yards. Two days later the two colts met yet again in the
Goodwood Cup over two and a half miles. Cossack raced prominently in the early part of the race but made no impression in the later stages and finished unplaced and tailed-off behind Van Tromp.
1849: five-year-old season Cossack reappeared as a five-year-old in the two and a quarter-mile
Tradesman's Plate at
Chester on 2 May. The handicap attracted a field of twenty-nine runners and Cossack was assigned 125 pounds, meaning that he had to concede weight to all but one of his opponents. The race was run at a slow pace and Frank Butler, riding Cossack, found himself trapped behind a "ruck" of horses on the final turn. Butler was forced to pull the horse to the wide outside and Cossack produced a strong finishing run, but failed by a "half neck" to catch Malton, a four-year-old to whom he was conceding thirty-one pounds. In June Cossack ran twice at Royal Ascot. In the
Gold Vase he was involved in a close finish,
dead-heating with the filly
Canezou for second place, a head behind the winner Glenalvon, after looking the likely winner. A day later, Cossack ran in the
Emperor of Russia's Plate, in front of a large crowd which included the
Queen and
Prince Albert. He finished third, beaten two lengths, behind Van Tromp and Chanticleer, in a race which, according to ''
Bell's Life'', created an "extraordinary degree of excitement".
1850: six-year-old season On his six-year-old debut, Cossack again traveled to Chester for the Tradesman's Plate, but on this occasion he finished unplaced under top weight of 128 pounds. At Goodwood on 1 August he finished a well-beaten third of the seven runners behind Canezou in the
Goodwood Cup. On 4 September Cossack appeared at
Warwick, where he finished fifth, carrying top weight of 120 pounds in the two-mile Leamington Stakes, giving at least twenty-seven pounds to the four horses who finished in front of him.
1851:seven-year-old season In July he ran for the third time in the Goodwood Cup and was narrowly defeated after his strong late run failed by a head against the filly Nancy, who was receiving twenty-four pounds according to
weight-for-age rules. After the race Pedley demanded a veterinary examination to determine the filly's true age: she was confirmed to be a three-year-old In August he appeared at
Brighton Racecourse where he finished second of the fourteen runners to Lord George in the Brighton Stakes. At Doncaster at the St Leger meeting in September, Cossack ran unplaced under top weight in the Great Yorkshire Handicap and was beaten the following day in a match race by the mare Maid of Masham.
1852:eight-year-old season On 23 June, Cossack, giving at least twenty-six pounds to his opponents, finished third to the three-year-old Stilton in the
Northumberland Purse. ==Assessment==