Billie Jean Moffitt was born in Long Beach, California, into a conservative Methodist family, the daughter of Betty (née Jerman), a housewife, and Bill Moffitt, a firefighter. Her family was athletic; her mother excelled at swimming, and her father played basketball and baseball, and ran track. Her younger brother, Randy Moffitt, became a Major League Baseball pitcher, pitching for 12 years in the major leagues for the San Francisco Giants, Houston Astros, and Toronto Blue Jays. She also excelled at baseball and softball as a child, playing shortstop at 10 years old on a team with girls 4–5 years older than her. The team went on to win the Long Beach softball championship.