Malcolm won the title of World Junior Athlete of the Year in 1998 and at the 1998 World Junior Championships, he won the 100m in 10.12 seconds and the 200m in 20.44 seconds, becoming the second sprinter to achieve this double at the Junior Worlds, after Ato Boldon. His winning time in the 100m set a new Welsh senior and British junior record, and his winning margin of 0.22 seconds was the largest in the history of the 100m at the Junior Worlds at the time. At that year's Commonwealth Games, his 20.29 for the silver medal was a European Junior and Welsh senior record.