Born in Monzón, Huesca, Martínez turned professional in 1988. In 1992, she 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.
In 1993, Martínez became the first Spanish woman since Lili de Alvarez in 1928 to reach the semifinals at Wimbledon, where she lost to Steffi Graf 7-6, 6-3. Martinez beat Graf for the first and only time in her career, at a tournament in Philadelphia. At the Italian Open, Martinez became the first Spaniard to win the tournament since de Alvarez in 1930.
In 1994, Martínez reached the Wimbledon singles final, where she faced 9-time former Wimbledon champion Martina Navratilova. Navratilova'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 clay courts. Martínez, however, won the match 6-4, 3-6, 6-3.
In 1995, Martínez was a semifinalist at all four Grand Slam tournaments and reached her career-high singles ranking of World No. 2.
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.
In 1998, 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 three hours and 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 Dokic). Martinez 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, at the age of 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.