She was trained in her youth by the mother of Mary Johanna Brown, who became Olympic champion in 1928. Mastenbroek won in 1934 at the European Championships in Magdeburg three gold medals and one silver medal. At the 1936 Summer Olympics in Berlin - at that time she was only 17 years old - she repeated this performance. She won the 100m freestyle, 400m freestyle and the 4 × 100 m freestyle relay. About 100 m backstroke she won behind her compatriot Nida Senff the silver medal ( Senff had the last turn happens not to conform and had to swim back a few feet, yet they won thanks to an exceptionally good performance on the last track ).
From 1937 Mastenbroek worked as a swimming instructor. She lost because of her status as an amateur, so she was no longer allowed to start. During her short career she had set up a total of nine world records, six in the backstroke and three in free style. She was inducted into the Hall of Fame of the international swimming sport in 1968, in 1997 she received the International Olympic Committee the Olympic Order.
- Gold: 100m backstroke ( 1:20,3 min)
- Gold: 400m freestyle ( 5:27,4 min)
- Gold: 4 × 100 m freestyle relay ( 4:41,5 min)
- Silver: 100m freestyle ( 1:08,1 min)
- Gold: 100m freestyle ( 1:05,9 min)
- Gold: 400m freestyle ( 5:26,4 min)
- Gold: 4 × 100 m freestyle relay ( 4:36,0 min)
- Silver: 100m backstroke ( 1:19,2 min)