Maertens won 50 times as a senior, including the national championship at Nandrin. In 1970 he came second to the Frenchman, Régis Ovion, in the world amateur championship. He competed in the individual road race at the 1972 Summer Olympics. He turned professional in 1972. The frame-maker Ernest Colnago and the former champion Ercole Baldini came to his house with an offer to join their SCIC team. They offered to support him in his last year as an amateur and then take him as a professional.