Her parents were Richard Chadwick, a police officer, and Mary Lacko, a homemaker who later operated a San Diego restaurant.

Starting at age eleven, she competed in rough water swims, winning an annual 2.5-mile race in the ocean off La Jolla, North of San Diego, 10 times in 18 years.

[3][2] Highly accomplished as a young competitive swimmer, Chadwick won the 100-yard backstroke event at the Junior AAU National Championship at age 14 in May 1933 with a time of 1:17, and the following August won the junior 500-meter freestyle event at the 1933 Southern Pacific National AAU Swimming and Diving Championships in Los Angeles.

[3][7] On July 4, 1933, Chadwick handily won the 600-yard Venice Roughwater Swim for Women with a time of 15 minutes flat.

[8] Chadwick swam in Southern California ocean races as an amateur for several decades, but had her heart set to swim across the English Channel.

[2] In 1950 Chadwick attempted to enter a 1950 Channel-swimming contest sponsored by the Daily Mail but was refused for lack of a significant reputation.

[2] On August 8, 1950, at the age of 32, she crossed the English Channel from France to England in 13 hours and 23 minutes, breaking the then-current women's record held by American swimmer Gertrude Ederle.

[2] In August 1957 she swam the Bristol Channel from Weston-super-Mare to Penarth Head, 11 miles in a record-breaking 6 hours 7 mins.

[3][12] In December 1951, Chadwick appeared as herself in Faye Emerson's Wonderful Town musical series on CBS television.