He joined the NL's Montreal Expos during the 1988 season and was traded to the AL's Seattle Mariners in late May of 1989. Johnson led the league in walks three years in a row, with 120 in 1990, 152 in 1991, and 144 in 1992.

He was also the league leader in strikeouts four years in a row, from 1992 through 1995. In the meantime, Johnson was improving his control and his results. In 1993, he had a 19-8 record with 3 shutouts and a 3.24 ERA. He was 13-6 in the strike-shortened 1994 season, and he went 18-2 with a 2.48 ERA in 1995, when he won his first Cy Young Award.

A back injury sidelined Johnson for the rest of the 1996 season after he'd won his first five decisions, but he came back strong in 1997. He set an American League record for lefties with 19 strikeouts against the Oakland As on June 24, and matched that against the White Sox on August 8.

During contract negotiatons before the 1998 season, the Mariners told Johnson that they couldn't afford to pay him what he deserved. He struggled to a 9-10 first half of the season and then was traded to the Houston Astros, where he won 10 of 11 decisions.

But the Astros didn't re-sign him either, and Johnson went into the free-agent market, finally signing a four-year, $53 million contract with the Arizona Diamondbacks of the National League.

With Arizona, Johnson has reached Hall of Fame levels, winning four straight Cy Young Awards while compiling an 81-27 record and notching 787 strikeouts while walking only 284 hitters.

However, he also set a record by losing 7 consecutive games in post-season play, beginning with the Astros in 1998. He finally broke that streak in 2001, when he beat the Atlanta Braves, 2-0, in the opening game of the National League Championship Series. He went on to win the deciding game, 3-2.

Johnson followed that with a 3-hit shutout of the Yankees in the second game of the World Series and winning the sixth game, 3-2. Then he came on in relief to pick up the victory in the seventh game, as well, giving the Diamondbacks the championship. He and right-handed starter Curt Schilling were named co-MVPS.

In 2003, a knee injury put Johnson on the disabled list for a large part of the season and rendered him ineffective when he was able to pitch. Many observers thought that his career was just about over. But Johnson came back strong in 2004 to re-establish himself as one of the best pitchers in baseball.

On May 18, 2004, Johnson became the oldest pitcher ever to throw a perfect game in the major leagues, striking out 13 while beating the Atlanta Braves, 2-0. Later in the season, he recorded his 4,000th career strikeout, becoming the fourth pitcher to reach that mark.