Deegan joined a Chicago fastpitch softball team for the American Softball Association National Tournament in 1943. She was soon signed by the league to play for the
Rockford Peaches. She played for the Peaches until midway through the season, when she was traded to the
Kenosha Comets, beginning a four-year stretch in which she played for six teams. In Deegan was with the Comets and
Springfield Sallies, and from through part of the season she played with the
Fort Wayne Daisies. She finished the season with the
Peoria Redwings before ending her career in , returning to the Rockford franchise. Her managers and coaches included former big-leaguers
Max Carey,
Jimmie Foxx,
Bill Wambsganss and
Marty McManus. Deegan also worked as the league's official photographer. As a pitcher, she gave up the winning run to the
Racine Belles in the final game of the 1946 Shaughnessy series but had a lifetime
earned run average of 2.36. Her career batting average was .260. In , the
Brooklyn Dodgers brought Deegan and two other women players to the team's spring training camp at
Bear Mountain, New York. She was given permission to travel with the team from manager
Leo Durocher but was not allowed to step on the field. However, after retrieving a foul ball during an exhibition game, one of the coaches gave her a
fungo bat and let her bat infield practice. Durocher was quoted as saying that "...if we run out of men, Millie will be the first on the team...if she were a man, she no doubt would have been a Dodger." ==Retirement==