Koosje van Voorn started swimming competitively as a teenager with Groningen club DZN. As a 17-year-old, she qualified for the Dutch Olympic team in Helsinki, where she was eliminated in the semi-finals of the 100 m freestyle. With the Dutch relay team, she earned a silver medal. As a student, she competed in the 1959 Universiade in Torino, winning a bronze in the 100 m freestyle. Also a member of the Dutch team in Italy was Piet ten Thije, whom she married in 1961. Ten Thije was a four-time Dutch champion (400 m, 1,500 m freestyle). Van Voorn and Ten Thije later lived in Germany and Indonesia on account of Ten Thijes work for Shell, but eventually they settled back in the Netherlands.