Rex Lassam finished fourth in the 1913 ASA 200 yards breaststroke but any chance of him improving on that placing was put on hold due to the outbreak of Wold War I. The next ASA Championships were after the War, in 1920, and this time, Lassam was the champion. A month earlier, after winning his heat in the 200 yard breaststroke in the Olympic trial at Southport, he was selected for the Antwerpen Games but finished fourth in both his 200 and 400 metre heats. In 1921 Lassam finished third in the ASA 200, his only other national podium finish.