Despite the end of the War, Doncaster racecourse was not ready to stage a classic in the autumn of 1945 and the St Leger was run over fourteen furlongs at York Racecourse. Prize-money, however, returned to pre-war levels and with £10,210 to the winner, the race was the most valuable run in Britain that year. Ridden by Tommy Lowrey, Chamossaire started at odds of 11/2 in a field of twelve runners with the Aga Khan's filly Naishapur starting 5/2 favourite. The race attracted a crowd estimated at 150,000 causing serious congestion on the roads leading to the racecourse: many spectators abandoned their cars and walked the final miles to the course. Stirling Castle took the lead after half a mile and led the field into the straight. Chamossaire took the lead a furlong from the finish and held off the late challenge of King George VI's colt Rising Light to win by two lengths, with Stirling Castle in third place. For his two remaining starts, Chamossaire returned to Newmarket and raced in all-aged competition. He finished second to Black Peter in the Jockey Club Stakes over fourteen furlongs and second by a neck to the filly Amber Flash in the two-mile Jockey Club Cup.