Commandeur started competing in athletics in 1973 and soon became a leading junior Dutch runner and javelin thrower. Next year she won national youth titles in the 800 m (indoor) and 300 m. In 1975 she finished second in the 400 metres senior Dutch championships. She later won the 800 m at the 1975 European Athletics Junior Championships in a time of 2:05.8. She shaved 0.2 s from it two weeks later in Aalst, setting a Dutch seniors record and a world youth record.