Jan Derksen was born at Geertruidenberg in central western Holland on 23 January 1919. He became interested in cycling after listening to radio reports of six-day races on the track. His father gave Derksen his first racing bike after he passed his last school exams at 15. He took out a racing licence with the Dutch cycling federation, the KNWU, and raced on the track at Alkmaar. A coach saw him when he was 18 and he joined the national team. He had worked until then in a cycle business.
At 19 he was third in the world amateur sprint championship, held in Amsterdam, and he became the only world champion in 1939 when the championship in Milan was abandoned because of the outbreak of the Second World War. His ride against the Italian, Astolfi, whom he beat in two rides, was the only event completed. He had been selected for the Dutch team for Olympic Games in Helsinki.