SAN FRANCISCO — Randy Johnson is retiring after 22 major league seasons.

The Big Unit, an overpowering left-hander who last June became the 24th pitcher to win 300 games, made the expected announcement Tuesday on a conference call.

"I really wanted to go out on my terms," Johnson said. "I just feel like there's not a lot more for me to do in this game. I just think it's a natural progression when you play this long. Eventually you have to say it's time."

A five-time Cy Young Award winner, the 46-year-old Johnson accomplished just about everything in his remarkable career that a player hopes for in baseball.

He owns a World Series ring and co-MVP honors, and was a 10-time All-Star. He threw two no-hitters, including a perfect game, and ranks second on the career strikeout list.

The 6-foot-10 Johnson finishes with a career record of 303-166 and 4,875 strikeouts in 4,135 1-3 innings for Montreal, Seattle, Houston, Arizona, the New York Yankees and San Francisco. His strikeouts are the most by a left-hander and second to Nolan Ryan's 5,714.

"It's all been a bit of a whirlwind. I never really got caught up in what I did," Johnson said. "I never really dwelled on my achievements. They're nice. Maybe now I'll be able to reflect on them."

Johnson overcame several injuries to keep pitching at a high level into his mid-40s. He said before last season ended that he looked forward to going home to Arizona and spending time with his family before making a decision about his future.

"It's taken this long into January because I definitely wanted to just kind of relax from the season being over and make sure I had a clear head when I made this decision, and that I would be making it wholeheartedly and would be sticking to it," he said.

Johnson went 8-6 with a 4.88 ERA in 17 starts and five relief appearances for San Francisco last season despite missing more than two months with a strained left shoulder that also had a tear in the rotator cuff. He returned in late September as a reliever, a role he couldn't see himself embracing in order to keep pitching.

His final strikeout came on the season's final day at San Diego, against Adrian Gonzalez to end the seventh inning. Johnson said he developed a better appreciation for relief pitchers last year.

"My 40s have really been learning years," he said. "The last five years of my career, there's been a lot there to sift through, a lot of ups and downs, some great moments in my career and some moments that got my head scratching."

Johnson came out of a game July 5 against Houston with an injury, the first serious shoulder problem of his career. He felt something in his arm on a swing during that start but initially tried to pitch through it. He then left the game after committing a throwing error in the fourth inning.

He was on the disabled list from July 6 to Sept. 16, marking the 10th DL stint of Johnson's career. He had four knee operations and three back surgeries, but worked his way back each time.

Johnson signed with the Giants to try to help them reach the playoffs, but they missed the postseason for a sixth straight year. San Francisco stayed in the NL wild-card chase well into September.

Pitching in San Francisco, Johnson was only about 40 miles west of where he grew up in Livermore.

He largely stayed to himself in his final season, insisting the 300-win milestone wasn't his top priority. Then on June 4 at Washington, he became the 24th pitcher in big league history to accomplish the feat.

And to think, early in his career Johnson was a shaggy-haired pitcher who lacked control of his blazing fastball.

"This isn't a tall man's sport — basketball is," he said.

But by the time he was done, he had a Hall of Fame resume.

Johnson pitched his first no-hitter in 1990, won 19 games with 308 strikeouts in 1993 and led the Mariners to their first playoff berth with an 18-2 record in 1995. He finished his 10-year stint in Seattle with a 130-74 record before being traded to Houston in 1998.

He signed as a free agent with the Diamondbacks before the following season, beginning one of the most dominating runs a pitcher has ever had. Johnson won the Cy Young in each of his first four seasons with Arizona, capturing the coveted pitcher's triple crown in 2002 with a 24-5 record, 2.32 ERA and 334 strikeouts.

His most memorable moments were in 2001, when he came out of the bullpen to beat the Yankees in Game 7 of the World Series to give the Diamondbacks the title. He went 3-0 in the Series, sharing the MVP award with Curt Schilling.

Johnson pitched a perfect game at age 40 against Atlanta.

Rickey Henderson certainly felt the heat before that. The Hall of Famer struck out in 30 of 59 at-bats against Johnson, making him the Unit's top victim.

Johnson didn't have as much success after leaving Arizona for the first time following the 2004 season. He won 34 games in two seasons with the Yankees, although the tenure was marred by a run-in with a camera man and postseason struggles.

He returned to Arizona in '07 and won just 15 games in two years while struggling with back problems.

"I never thought I was going to play this long. I'm blessed that I did," Johnson said.

The Big Unit added that he plans to coach someday. He has a one-year service agreement to work in some capacity for the Diamondbacks.