Competing in gymnastics as a teenager, François Claessens was educated as a diamond cutter. He served in the Belgian army in the run-up to the Antwerp Olympics, but was allowed to train for the Games during his service. The training paid off, as the Belgian team placed second in the team all-around (European system). Claessens remained active in gymnastics, competing internationally again at an international concours in Brno (Czechoslovakia). He later operated a diamond cutting shop in Antwerpen.