King was born in Long Beach, California, into a conservative Methodist family, the daughter of Betty (nee Jerman), a housewife, and Bill Moffitt, a firefighter. Billie Jean's family was athletic. Her mother excelled at swimming, her father played basketball, 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.

When the open era began, King campaigned for equal prize money in the men's and women's games. In 1971, with ideas for the formation of an eight player women's group from her husband, Larry King, financial backing of World Tennis magazine founder, Gladys Heldman, and the sponsorship of Virginia Slims Chairman Joe Coleman, King became the first woman athlete to earn over US$100,000 in prize money; however, inequalities continued. King won the US Open in 1972 but received US$15,000 less than the men's champion Ilie Nastase. She stated that she would not play the next year if the prize money were not equal. In 1973, the US Open became the first major tournament to offer equal prize money for men and women.  King led player efforts to support the first professional women's tennis tour in the 1970s called the Virginia Slims, founded by Gladys Heldman and funded by Joseph Cullman of Philip Morris. Once the tour took flight, King worked tirelessly to promote it even though many of the other top players were not supportive. "For three years we had two tours and because of their governments [Martina] Navratilova and Olga Morozova had to play the other tour. Chris [Evert], Margaret [Court], Virginia [Wade], they let us do the pioneering work and they weren't very nice to us. If you go back and look at the old quotes; they played for the love of the game, we played for the money. When we got backing and money, we were all playing together - I wonder why? I tried not to get upset with them. Forgiveness is important. Our job was to have one voice and win them over."  In 1973, King became the first President of the women's players union - the Women's Tennis Association. In 1974, she, with husband Larry King and Jim Jorgensen, founded womenSports magazine and started the Women's Sports Foundation. Also in 1974, World TeamTennis began, founded by Larry King, Dennis Murphy, Frank Barman and Jordan Kaiser. She became league commissioner in 1982 and major owner in 1984.  King is a member of the Board of Honorary Trustees for the Sports Museum of America, which opened in 2008. The museum is the home of the Billie Jean King International Women's Sports Center, a comprehensive women's sports hall of fame and exhibit.

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