Clendenon had a controversial year prior to his arrival in New York. In January 1969 he was left unprotected by the Pirates and was selected in the expansion draft by the Montreal Expos. He was then traded to Houston for Rusty Staub. But Clendenon wasn't happy about going to Houston, he had had a falling out with manager Harry Walker in their Pittsburgh days. Instead of going to Houston he announced his retirement. Commissioner Bowie Kuhn stepped in and told the teams to work something out. Mostly due to the fact, Montreal had promoted Rusty Staub as the face of their new organization. The Expos sent Jack Billingham and Skip Guinn to Houston and Clendenon continued to play for Montreal, in 38 games he hit .240 , with 4 HRs & 14 RBIS.
On June 15th Clendenon was traded to the Mets for Steve Renko & Kevin Collins. As soon as he arrived in New York he made a difference, scoring two game winning runs in his first two games. He hit 12 HRs with 37 RBIs in 72 games, platooning at first with Ed Kranepool. Clendenon gave the Mets power against lefties and more strength off the bench. They overcame the 9 ½ game deficit, and on September 24th Clendenon's three run HR helped the Mets clinch the NL East title.
In the 1969 World Series, Clendenon was the Series MVP. He set a record with three HRs in a five game series, and his home runs in Games Two, Four and Five meant the winning run each time. He hit .357 with 3 HRs, a double, 2 walks, and 4 RBIs.
In retrospect his teammates raved about his presence on the club: Tug McGraw said "he was probably the key to our whole season,". Wayne Garrett called Clendenon the last ingredient the team needed. Art Shamsky called him the catalyst and Bud Harrelson, said "We never had a three-run homer type of guy, yet he was humble, never cocky, our MVP.”
Tom Seaver remembers his wife spotted Clendenon in the hotel lobby when he joined the team."I know who you are," Nancy said. "Donn was wearing an island shirt and vest, he turned to her and suavely kissed her hand. Nancy thought Donn was charming; he knew she was my wife and put on a little show. "It's great to be a Met" he said.