Born in Rotterdam, she started swimming coached by "Ma" Braun, who had coached her daughter to an Olympic gold medal in 1928. In 1934, Mastenbroek won three gold medals and a silver at the European Championships.
She repeated that performance at the 1936 Summer Olympics, winning the 100 m freestyle, 400 m freestyle and the 4 x 100 m freestyle. In the 100 m backstroke, she finished second behind teammate Nida Senff[?]. At that time, she was still only 17 years old.
The next year, she became a swimming instructor, thereby becoming a professional and ineligible for competition.
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