From left, the Mets' Bud Harrelson, Tom Seaver, Jerry Koosman...

From left, the Mets' Bud Harrelson, Tom Seaver, Jerry Koosman and Ed Kranepool celebrate clinching the National League Eastern Division on Sept. 24, 1969. Credit: SPORTS ILLUSTRATED/GETTY IMAGES/Herb Scharfman

Fifty years ago on Tuesday -- Sept. 24, 1969 -- one of the most improbable events in human history occurred.

The Mets clinched their first postseason berth by winning the National League East title in a 6-0 victory over the St. Louis Cardinals before 54,928 screaming fans at Shea Stadium.

The Mets, who were born in 1962, averaged 105 losses in their first seven seasons, including a record 120 in their first season. In 1969, the Mets, under manager Gil Hodges,

went 100-62. As you know, they went on to win the World Series and became one of the most celebrated sports teams ever.

But the first step was to win the first NL East title as baseball had split the leagues into two divisions beginning that season.

“It was the first time we ever won anything,” said first baseman Ed Kranepool, who joined the Mets during their first season in 1962. “You couldn’t wait to get to the ballpark to play the games. It was the first time we had any meaningful games in September. It’s exciting to be involved in a pennant race and certainly the situation where the Mets had lost more than 700 games in the first seven years and then you’re in it, it was exciting for us and our fans. They were behind us all the way and they got louder and louder as the year went on.”

The Mets trailed the division-leading Cubs by 10 games as late as Aug. 13. They went 38-11 from that point and won the East by eight games.

“We knew what had to be done,” third baseman Wayne Garrett said. “We had to win our division. We didn’t have wild cards like you do now. We just got on a roll the last two months of the season. We just knew if we could just keep winning games, we could come in in first place in the division. And we did.”

By Sept. 24, the formal clincher was a foregone conclusion. The Mets came out on that Wednesday night in Flushing and scored five runs in the first inning against future Hall of Famer Steve Carlton, with Donn Clendenon hitting a three-run homer and Ed Charles following two batters later with a two-run shot.

Carlton was knocked out after one-third of an inning. Clendenon hit a solo homer off Dave Giusti in the fifth to make it 6-0. That’s all Gary Gentry would need as the righthander threw a four-hitter.

“Our pitching staff was really in a groove coming down the stretch,” Kranepool said. “We had four guys throwing well, including Gary Gentry. He had as good a stuff as anybody on the staff.”

The final outs came when Gentry got Joe Torre to hit into a 6-4-3 double play. The Mets had tried to trade for Torre before the season but couldn’t come to an agreement with the Braves.

“The Braves wanted Gary Gentry and Nolan Ryan —

they wanted to get into those young arms,” recalled outfielder Ron Swoboda. “The [Mets] general manager, Johnny Murphy, I think was like, ‘We don’t know what this team can do. We don’t know if Torre is the missing piece or not.’”

The Mets held on to their young arms, and Torre was shipped to the Cardinals for Orlando Cepeda. The Mets waited until June 15 to acquire Clendenon from Montreal to be a big righthanded bat and Kranepool’s platoon partner.

In addition to his clinching-game heroics, Clendenon went on to become the World Series MVP. But that’s a story for another day.

First, Mets fans got to savor the East crown and look forward to the first NL Championship Series against Atlanta, the NL West champions (in a feat of geographical tomfoolery that wasn’t rectified until the Braves joined the NL East in 1994).

Before 1969, when there were no divisions, the regular-season champions in the AL and NL would advance directly to the World Series. Baseball expanded to 24 teams in 1969 by adding Montreal and San Diego in the NL and Kansas City and Seattle in the AL and also added a round of playoffs.

“It was strange because nobody knew anything about it,” Kranepool said. “You’re in first place, but you’re not going to the World Series. Everything depends on a five-game series to get to the World Series. Our journey had just begun.”

So it had. To be continued . . .

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