Rosenfeld was born in Dnepropetravsk, Russia on December 28th, 1904. Her family then immigrated to the small town of Barrie, Ontario when she was just an infant.

As a child growing up in Barrie, she ran her first sprint out of necessity. At a town picnic she and a sister lost the money they had brought to buy their meal. Luckily a race had been organized for children, with a box lunch as prize. Rosenfeld entered and won.

By the time she was a teenager, Rosenfeld had become well known locally as an outstanding, dedicated athlete. She led her high school basketball team at Barrie Collegiate Institute to a League title, and excelled in track as well as in her favorite sport, ice hockey. Friends had even begun calling her "Bobbie" for her short bobbed hair, cut to stay out of the way during competition. And while sexist opinions on women in sports were common throughout Rosenfeld's childhood, her family supported and encouraged her. Brushing aside assertions that strenuous exercise was damaging to the feminine form, her father was always wildly cheering from the stands at Bobbie's games and races. When the family moved to Toronto in 1922, Rosenfeld was ready to make her mark on the city's growing women's sports scene.