Gilberte Mortier learned to swim at a young age when her mother, the noted French feminist Renée Mortier, took her to the Libellule de Paris swimming club. Mortier became part of the national swimming team and competed in the 100 metres backstroke at the 1922 Women’s Olympic Games in Monaco. Two years later she had broken the French record for the 200 metres freestyle, earning herself a place on the Olympic team. At the Paris Games she finished fifth in the 4 × 100 metres freestyle relay, in addition to swimming in the 400 metres freestyle. In 1925 Mortier finished third in the 1,000 metres freestyle at the French championships.