On her eighteenth birthday, Evert turned professional, beginning her career on the United States Lawn Tennis Association (USLTA) circuit. She won their championship from 1975-78, and in 1980 and 1982. Though she was young, tennis was the defining aspect of her life. She told Sally Jenkins of Sports Illustrated in 1992, "I was very insecure when I was young. I was shy and introverted. When I went out on the tennis court, I could express myself. It was a way of getting reactions from people, like my father. I really admired my dad and put him on a pedestal, and I wanted his attention. Whether it's ego or insecurity or whatever, when you start winning and getting attention, you like it, and that feeling snowballs. You start to feel good about yourself. You feel complete and proud of yourself." As Evert told Christopher Whipple of Life in 1986, "When I was younger I was a little robot: Wind her up and she plays tennis."
Evert dominated on the court from earliest days as a professional. From August 1973 through May 1979, she did not lose one match on clay courts. This was 125 straight matches on the surface best suited to her game. During this span, she won the French Open, which was played on clay, in 1974, 1975, and 1979. It was in 1974 that she began to blossom as a professional. In addition to winning the French Open, her first Grand Slam win, she won Wimbledon, and the Italian and Canadian championships.
Evert played many of these early matches on television, the beginnings of when some women's sports began to air. One aspect of the 1974 win at Wimbledon was made for television. At the time, she was engaged to men's tennis star Jimmy Connors, who won the men's singles title. It was dubbed the "summer of love." Evert's first career year was also highlighted by the fact that she was ranked number one on the USLTA, the youngest woman to be number one in decades.
After her 1974 wins at the French Open and Wimbledon, Evert won at least one Grand Slam women's singles title until 1986. In the mid-1970s, she won at least two, as well as a number of women's doubles titles, and was ranked number one in the world from 1975-77. In 1976, Evert was named Sports Illustrated's sports-woman of the year. In 1977, she was named the Associated Press's Female Athlete of the Year. In this time, Evert also played World Team Tennis for Phoenix, from 1976-77. She later won a World Team Tennis championship with Los Angeles.