NEW YORK — Mike Mussina took a secret to the ballpark every day this season. Maybe that’s why he was so successful on the mound, so jovial in the clubhouse.

From Day One of spring training, he knew this was his final year — even if it meant giving up bids for 300 wins, a World Series ring and a better shot at the Hall of Fame.

The New York Yankees pitcher walked away from baseball Thursday after his only 20-win season, a month shy of his 40th birthday with a still-potent right arm.

“I don’t have any regrets with what I decided. This is the right time,” Mussina said on a conference call.

“I don’t think there was ever a point where I looked around and said, ‘You know what, I’m going to change my mind,’” he said. “It was like the last year of high school. You know it’s going to end and you enjoy the ride.”

Mussina finished 270-153 with a 3.68 ERA in 18 seasons with Baltimore and New York. A thinking man’s pitcher who relied on sharp control and did more than overpower hitters, he ranks 32nd on the career wins list and 19th in strikeouts with 2,813.

His dad and brother tried to talk him out of retirement and Yankees general manager Brian Cashman called to ask about his future. Instead, “Moose” became the first healthy pitcher to leave on his own accord following a 20-win season in more than a century.

Sandy Koufax left the Dodgers after 1966 because of agonizing elbow pain, Eddie Cicotte and Lefty Williams got tossed in the Black Sox scandal. Henry Schmidt won 22 for Brooklyn in 1903 as a rookie, then decided to go West and return to the Pacific Coast League.

“I love baseball. I love playing, I love pitching,” Mussina said. “This is good for me. This is it.”

On Mussina’s last day as a big leaguer, he pitched six shutout innings at Fenway Park to beat Boston and wind up 20-9 — it made him the oldest first-time 20-game winner ever.

“It was a nice way to finish,” he said.

As for what’s next, “I don’t really have any plans,” he said.

A man of diverse interests, there’s no telling whether he’ll get to enjoy the things he sometimes talked about — going to the Daytona 500, seeing his favorite Irish setters in the Westminster Kennel Club dog show.

A few hours before his official announcement, Mussina shoveled snow and cleaned closets at his home in Montoursville, Pa. He likes small-town life, often coached basketball and football at the local high school and intends to spend a lot of time watching his 10- and 5-year-old sons play ball.

In five years, Mussina will become eligible for the Hall of Fame, and it figures to be a lively debate. He acknowledged there are solid arguments on both sides.

“It’s not my decision to make,” he said.

Mussina never won a Cy Young Award or a World Series championship and was a 20-game winner just on the final day of his career. He was 7-8 with a 3.42 ERA in postseason play.

He was amazingly consistent — after going 4-5 as a rookie with Baltimore in 1991, he became the only AL pitcher to reach double figures in wins for 17 straight years. He was a five-time All-Star and won his seventh Gold Glove earlier this month.

Tall and slim, Mussina spent his whole career pitching in the bruising AL East, smack in the Steroids Era.

His best outing came in 2001 at Boston, when he came within one strike of a perfect game before Carl Everett singled. Mussina’s proudest moment came in Game 7 of the 2003 ALCS, when he made the first relief appearance of his career and threw three scoreless innings against the Red Sox.

“My numbers match up well with guys who are in the Hall of the Fame,” he said, adding other pitchers have won more games and been left out.

Said Yankees captain Derek Jeter: “Moose’s accomplishments in the game over the last 18 years represent a Hall of Fame player.”

Mussina said he couldn’t envision himself working in baseball in the foreseeable future, though he was open to someday helping out young pitchers.

Money isn’t an issue. He left Baltimore to sign an $88.5 million, six-year contract as a free agent with the Yankees before the 2001 season and finished with a $23 million, two-year deal.

Mussina graduated early from Stanford with an economics degree — he won the College World Series with the Cardinal — and came to the Yankees with a reputation as an ace pitcher with a prickly personality. He softened over the years, and by the end became a chatty clubhouse spokesman who kept reporters chuckling with his witty observations and thoughtful answers.

Mussina frequently sat in front of his locker doing crossword puzzles. In fact, he appeared in the 2006 documentary “Wordplay” and kept a thesaurus in his stall for knotty problems.

No words, however, could change his mind on calling it a career. Mussina said he decided last January that he had one more season.

“Good or bad, this is going to be the last year,” he told himself. “That’s just how I felt.”

Mussina said most of his teammates knew his plans. More relaxed, he called it a fun year.

Now, he said, it was time to stay home. Spring training would have to come and go.

“I think February is going to be a little strange,” he said. “It’ll be different. It’ll be weird.”