Through being trained by Gordon Elliott, Don Cossack won his first four races in the Irish Form Book Future Champions INH Flat Race and the INH Stallion Owners European Breeders Fund Maiden Hurdle at Navan, as well as the Paul Limburn 40 With Friends INH Flat Race at Naas and the William & Gladys Bourke INH Flat Race at Fairyhouse. He fell in his third entry at Navan in the Novice Hurdle, responding immediately after with wins in the WB Gavin & Co European Breeders Fund Beginners Chase at Galway and the Bar One Racing Drinmore Novice Chase at Fairyhouse, finishing second in the Racing UK Moscow Flyer Novice Hurdle and the Dobbins Outdoor Catering Florida Pearly Novice Chase at Punchestown, as well as in the Dr PJ Moriarty Novice Chase at Leopardstown.
A topsy-turvy start left a lot to be desired, with each-way finishes being the only consolation, but there was more disappointment in his first entry into Cheltenham Festival, as he fell for a second time. His first entry into Aintree Festival saw more promise, resulting as the runner-up in the Betfred Mobile Mildmay Novices’ Chase, then only reaching fourth in the Growise Champion Novice Chase, but managing four wins on the bounce in the Star Best For Racing Coverage Chase at Punchestown, the John Durkan Memorial Punchestown Chase, the Powers Irish Whiskey Chase at Down Royal, and the Ladbrokes Ireland Kinloch Brae Chase at Thurles. It was the perfect way of preparing for the second attempt at Cheltenham Festival, where he finished third in the Ryanair Chase.
The next race saw an incredible win in the Betfred Melling Chase at the 2015 Aintree Festival, keeping this fine form in winning the Bibby Financial Services Ireland Punchestown Gold Cup, the Irish Daily Star Chase at Punchestown, the JN Wine Champion Chase at Down Royal, falling in the William Hill King George VI Chase at Kempton, but winning again in the Ladbrokes Ireland Kinloch Brae Chase at Thurles. Running in the Timico Cheltenham Gold Cup Chase at the 2016 Cheltenham Festival saw an opportunity for Don Cossack to reach his full potential, winning the Gold Cup at a starting price of 9/4, in what ended up being his last ever professional race.