Known as the Fulton Flash, Stephens had won nine Amateur Athletic Union track-and-field titles by the age of 18. At the 1936 Olympic Games, Stephens won the 100-metre dash in 11.5 sec. She was also a member of the U.S. 4 × 100-metre relay team that won a gold medal. The U.S. team had a pair of 100-metre gold medalists, with 1928 champion Elizabeth Robinson joining Stephens, but for most of the race they trailed the Germans, who had set a world record in a qualifying round. The Germans dropped the baton and were disqualified, and the U.S. squad edged out Great Britain by less than a second. Adolf Hitler was said to be so impressed by Stephens that he invited her to his private box.

After winning three more U.S. national titles (50 metres, shot put, and 200 metres), Stephens retired from competitive track. During her 30-month career, she competed in more than 100 races, winning every one. She and Jesse Owens (Owens, Jesse) headlined a tour before Stephens moved on to briefly play professional basketball and softball. During World War II she served with the U.S. Marine Corps. By the 1980s Stephens returned to competitive track-and-field in senior events, maintaining her perfect record.