Born in San Diego, Chadwick had her first swimming competition win at the age of ten, and at the age of eleven competed in her first "challenging" competition, a rough water swim. She placed fourth in the event.
Chadwick's biggest contribution to swimming history occurred on August 8, 1950, when she crossed the English Channel in 13 hours and 20 minutes, breaking the then-current world record held by American swimmer Gertrude Ederle. One year later, Chadwick crossed the English Channel yet again, from England to France; this time, in 16 hours and 22 minutes, thus making her the first woman to swim the English Channel in both directions, and setting a record for the England-France journey.
In 1952, Florence was the first woman to attempt to swim the 26 miles between Catalina Island and the California coastline. As she began, she was flanked by small boats that watched for sharks and were prepared to help her if she got hurt or grew tired. After about 15 hours a thick fog set in. Florence began to doubt her ability, and she told her mother, who was in one of the boats, that she didn’t think she could make it. She swam for another hour before asking to be pulled out, unable to see the coastline due to the fog. As she sat in the boat, she found out she had stopped swimming just one mile away from her destination.
Two months later, Chadwick tried again. This time was different. The same thick fog set in, but she made it because she said that she kept a mental image of the shoreline in her mind while she swam.