Billie Jean Moffitt was born on 22 November 1943 in Long Beach, California, the eldest child of Bill and Betty Moffitt, in an athletic family; her father had played college basketball. At 11 she bought her first racquet with money saved from odd jobs and took free lessons at Houghton Park near her home. After one of those sessions she told her mother she was going to be No. 1.
Her first Wimbledon title came in 1961, in the women's doubles with Karen Hantze. The first of her six Wimbledon singles titles followed in 1966, and she won it again in 1967, 1968, 1972, 1973 and 1975. Her game was aggressive and forward-moving, and it was at its most dangerous on grass. In all she won 39 major titles, 12 in singles, 16 in doubles and 11 in mixed, a record 20 of them at Wimbledon, and she completed the career Grand Slam in singles.
Her fight off the court changed the sport. At some tournaments the men were paid twelve times as much as the women. After winning the 1972 US Open she threatened to stay away the next year unless the prize money was equal, and in 1973 the US Open became the first major to pay men and women the same. On the eve of Wimbledon that year, at a meeting of about 60 players in London's Gloucester Hotel, she led the founding of the Women's Tennis Association and became its first president. It took 34 years for all four majors to follow the US Open; Wimbledon was the last, in 2007.
On 20 September 1973, aged 29, she played the 55-year-old former Wimbledon champion Bobby Riggs in the "Battle of the Sexes" at the Houston Astrodome. She won 6–4, 6–3, 6–3 and the $100,000 prize, in front of a television audience estimated at 90 million. Sports Illustrated made her its Sportsperson of the Year, the first woman to receive the honour. In 1974 she co-founded World TeamTennis and created the Women's Sports Foundation. In 2006 the home of the US Open was renamed the USTA Billie Jean King National Tennis Center, the first major sports venue named after a woman.