He soon moved to Perth, where he started swimming in the Swan River by the age of three, and in 1935 to Melbourne.

The following year, he was selected for the London Olympics, where he claimed bronze in the men's 400-metre freestyle behind the American duo of Bill Smith and Jimmy McLane.

McLane's coach, Bob Kiphuth was so impressed by Marshall's performance that the same year he arranged for Marshall to study at Yale University and swim for the Yale Bulldogs swimming and diving team.

Amateur Athletic Union (AAU) Indoor Championships he collected the 220-yard, 440-yard, 880-yard, 200-metre, 400-metre and 1-mile titles.

[1] In 1954 he returned to Australia, where he worked for the tyre company of the Australian former swimmer Frank Beaurepaire.