Born in Natal, Walker, the 1907 South African Champion, was not among the big favourites for the 100 metres at the 1908 Summer Olympics. He even had trouble getting to London, as he lacked the necessary finances until a Natal sportswriter collected funds to support Walker's travel. In England, he was coached by Sam Mussabini, later also the coach of Harold Abrahams.
Several of the big names did not qualify for the final, but Walker did. His first round was a relatively easy victory run in 11.0 seconds. In the second round, Walker edged out William W. May of the United States while tying the Olympic record of 10.8 seconds. This qualified him for the final. There he competed against three North Americans, including James Rector of the United States, who had equalled the Olympic Record in both of the qualifier rounds. Walker beat Rector in the final by about a foot and half.
Walker is still the youngest winner of the Olympic 100 metres as of 2016 (at 19 years and 128 days).