Martinez's career started with the Los Angeles Dodgers in 1992, as a relief pitcher. After the 1993 season, he was traded to the Montreal Expos for Delino DeShields, and became one of the top pitchers in baseball. In 1997 he posted a 17-8 record for the otherwise mediocre Expos, with a 1.90 ERA, 305 strikeouts and 13 complete games pitched, and won the National League Cy Young Award.

Martinez was traded to the Boston Red Sox in November 1997 for Carl Pavano and Tony Armas Jr, and was soon signed to a six-year, $75,000,000 contract by the Sox, at the time the largest ever awarded to a pitcher. In 1999 he enjoyed one of the greatest pitching seasons of all time, finishing 23-4 with an ERA of 2.07 and 313 strikeouts, as he won his second Cy Young Award, this time in the American League, and was second in the league's most valuable player ballot. The 1999 MVP vote was controversial as Martinez received the most first-place votes, but was totally omitted from the ballot of two sportswriters who believed pitchers were not sufficiently all-around players to be considered.

In the 1999 playoffs, though hampered by an injury, Martinez won game five after appearing in relief and pitching six shut-out innings against the Cleveland Indians. In the American League Championship Series, he pitched seven shut-out innings to beat the New York Yankees in game 3, handing them their only loss of the postseason.

Martinez's strikeouts and win count were slightly down in 2000, but he posted a stellar 1.74 ERA and won his third Cy Young Award. He was injured for much of 2001 as the Red Sox slumped to a poor finish. He rebounded in 2002 to post a 2.26 ERA with 20 wins, but that season's American League Cy Young award went to Barry Zito of the Oakland A's who had three more wins, despite a higher ERA, fewer strikeouts, and a lower winning percentage. Martinez became the first pitcher in history to lead his respective league in these categories and not win the Cy Young award.

Martinez has come about as close to throwing a perfect game as possible without actually getting credit for it. On 1995 June 3, while pitching for the Montreal Expos, he retired the first 27 San Diego Padres hitters he faced to sail through nine innings of perfect pitching. However, the game was still tied 0-0 at that point and the game went into extra innings, and Martinez surrendered a double to the 28th batter. According to Major League Baseball rules, that meant that Martinez accomplished neither a perfect game or a no-hitter.

Martinez is unusual for a power pitcher as he is 5'11" and 170 pounds, small by modern-day standards. His pitches include a 95-plus mph fastball that has a lot of movement, a deceptive change-up, and a very effective curveball.

Martinez's brother Ramon Martinez was also a Major League pitcher and the brothers have twice been teammates, with the Los Angeles Dodgers during 1992-1993 and with the Boston Red Sox during 1999-2000.

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