Dutch swimmer, who at the 1936 Olympics in Berlin became the first female athlete to win four medals at a single Games.

Mastenbroek swam in the canals of Rotterdam, Netherlands, to train for distance races and in indoor pools to train for sprint races. In 1934 she won the European 100-metre backstroke and 400-metre freestyle championships. At the 1936 Olympics, Mastenbroek came from behind to win the 100-metre freestyle with a closing burst of speed. In the 400-metre relay the Dutch team trailed for half the race until Mastenbroek's teammate Willy den Ouden took the lead; a final surge by Mastenbroek won her team another gold medal. Her third gold medal came in the 400-metre freestyle, in which Mastenbroek trailed Ragnhild Hveger of Denmark for most of the race, coming from behind once again in the final 25 m to win her third gold medal. Although Mastenbroek held the world record (1 minutes 15.8 seconds) in the 100-metre backstroke, she only won a silver medal in that event.

Mastenbroek also set world records in the 440-yard freestyle (5 minutes 29.1 seconds), 150-yard backstroke (2 minutes 49.6 seconds), and 400-metre backstroke (5 minutes 48.8 seconds). In 1968 she was inducted into the International Swimming Hall of Fame.