At the age of 17, Mitchell began playing for the Engelettes, a women's team in Chattanooga, Tennessee, and went on to attend a baseball training camp in Atlanta, Georgia. In doing so she attracted the attention of Joe Engel, the president and owner of the Chattanooga Lookouts, who was known for using publicity stunts as a way to draw crowds during the Great Depression. Seeing Mitchell as an opportunity to draw attention to the Lookouts, he signed her to the team on March 25, 1931. She appeared in her first professional game on April 2 becoming only the second woman to play organized baseball behind Lizzie Arlington who pitched for the Reading Coal Heavers against the Allentown Peanuts in a Minor league game in 1898.
The New York Yankees and the Chattanooga Lookouts were scheduled to play an exhibition game in Chattanooga, Tennessee, on April 1, 1931, however due to rain the game was postponed until the next day. Seventeen-year-old Mitchell was brought in to pitch during the first inning by Lookouts manager Bert Niehoff after the starting pitcher, Clyde Barfoot, gave up a double and a single. The next two batters were Babe Ruth and Lou Gehrig. After taking a ball, Ruth swung and missed at the next two pitches. Mitchell's fourth pitch to Ruth was a called third strike. Babe Ruth glared and verbally abused the umpire before being led away by his teammates to sit to wait for another batting turn. The crowd roared for Jackie. Babe Ruth was quoted in a Chattanooga newspaper as having said:
