In the final she was in fifth place at the 50m turn and was still only third with ten metres to go. However, her furious finishing strokes gave her a dramatic victory. Three days later, she earned a silver medal in the 100m backstroke and she might have done even better had she not become entangled at one point in the lane ropes. The following day she swam the anchor leg for the Dutch team in the 4x100m freestyle relay. In the battle for first place against Germany, the race was still in doubt with twenty metres left, at which point Mastenbroek sprinted to victory. The day after that, she competed in her fourth final in five days, the 400m freestyle.P
Before the race, one of the others swimmers, Ragnhild Hveger, shared a box of chocolates with the other competitors, but did not offer any to Mastenbroek. Mastenbroek promised herself that she would take revenge in the pool. Hveger led throughout the race, but with 25m to go, Mastenbroek pulled ahead and won by one metre. She was the first woman in any sport to earn four medals in one edition of the Games. She donated one of her gold medals to raise funds for the construction of a village for disabled people.