Martínez used extreme topspin on her forehand and slower topspin and slice on her backhand. She was a patient baseliner who won matches by disrupting her opponents' rhythm through changes of spin, pace, depth, height, and angle. She was known for expending "plenty of time and energy securing the ball with which she had just won the previous point so she could serve it again,"1 a major irritant to her opponents.
Born in Monzón, Martínez turned professional in 1988. At the age of just 17, she reached the fourth round at the French Open in her third professional tournament. She upset Lori McNeil en route. In 1989, her breakthrough year, Martínez beat Gabriela Sabatini to win the title at Tampa and won two other tournaments. She also reached the quarterfinals of the French Open, losing to Steffi Graf. She finished the year World No. 7. In 1990 and 1991, Martínez won a further six titles and again reached the quarterfinals at the French Open both years (losing to Graf in 1990 and Monica Seles in 1991).
The following year, Martínez was a silver medalist in doubles at the Olympic Games in Barcelona (partnering Arantxa Sánchez Vicario) and the runner-up in women's doubles at the French Open. Once again, she was a quarterfinalist at the French Open, losing a tight match with Sabatini. In 1993, Martínez became the first Spanish woman since Lilí de Álvarez in 1928 to reach the semifinals at Wimbledon, where she lost to Graf 7–6, 6–3. Martínez beat Graf for the only time in her career, at a tournament in Philadelphia. At the Italian Open, Martínez became the first Spaniard to win the tournament since de Álvarez in 1930. She again reached the quarterfinals at the French Open for the fifth year in a row, losing a 2 hour, 45 minute battle with Anke Huber 6–7, 6–4, 6–4.
Martínez reached the Wimbledon singles final in 1994, where she faced nine-time former Wimbledon champion Martina Navrátilová. Navrátilová's last Wimbledon triumph had come four years earlier, but many observers felt that the 37 year-old Czech-born American was the favourite going into the match given her long track record of success on grass courts, whereas Martínez's most significant tournament victories up to that time had been on slower-playing surfaces, particularly on clay courts. Martínez, however, won the match 6–4, 3–6, 6–3 and became the first Spanish woman ever to win Wimbledon. In 1995, Martínez was a semifinalist at all four Grand Slam tournaments and reached her career-high singles ranking of World No. 2. At Wimbledon, she beat Sabatini in the quarterfinals before losing to Sanchez Vicario in the semifinals.
In 1996, Martínez became the only player to win the Italian Open singles title four consecutive years. She also partnered Sánchez Vicario to claim a women's doubles Olympic bronze medal in Atlanta, Georgia. Two years later, Martínez reached her second career Grand Slam singles final. She was defeated in the final of the Australian Open by Martina Hingis 6–3, 6–3. She also helped Spain win the Fed Cup that year, beating Patty Schnyder of Switzerland 6–3, 2–6, 9–7 in 3 hours, 19 minutes in the final.
Martínez reached the final of the French Open in 2000, where she lost to Mary Pierce 6–2, 7–5 after beating Sanchez Vicario in a semifinal. She also won the German Open, beating Hingis in a semifinal and Amanda Coetzer in the final. In 2001, Martínez was a runner-up in the women's doubles at the French Open (partnering Jelena Dokić). Martínez also reached the quarterfinals at Wimbledon for the first time in six years but lost to Justine Henin of Belgium.
Martínez won her second Olympic silver medal in the women's doubles in 2004 in Athens, Greece (partnering Virginia Ruano Pascual). In 2005, Martínez won her first singles title in five years at Pattaya, Thailand, bringing her career total to 33 top-level singles titles, 9 of which were Tier I events, and 13 doubles titles. On April 15, 2006, aged 33 and after 18 years of playing professionally, she announced her retirement, having won more professional singles tournaments than any other Spanish female tennis player.