[3][4] He enrolled at San Francisco State University and, after one year, signed with the New York Mets as an amateur free agent in 1963.

[6][7] Weighing only 165 pounds (75 kg), Harrelson received the nicknames of "Twiggy", "Mighty Mouse" and "Mini Hawk" from his teammates.

[2] On May 28, 1969, after a five-game losing streak that saw the Mets fall into fourth place in the newly aligned National League East, Harrelson hit an RBI single that won the game,[9] beginning an 11-game team winning streak[10] and a record of 82–39 over the rest of the season.

On September 24, the team clinched the NL East with a 6–0 victory over Steve Carlton and the St. Louis Cardinals.

"[20] Harrelson was confronted by Reds second baseman Joe Morgan during pregame warmups for Game 3 and warned that Pete Rose was unhappy with the quote.

The game was nearly forfeited when the Shea Stadium crowd threw objects at Rose, causing manager Sparky Anderson to remove the team from the field until order was restored.

Mets manager Yogi Berra and players Willie Mays, Tom Seaver, Cleon Jones and Rusty Staub walked to left field to attempt to calm the fans.

[22] After the Mets reacquired former #1 overall pick Tim Foli, Harrelson was traded to the Philadelphia Phillies for Fred Andrews and cash on March 23, 1978.

[27] During the 1983 season, Harrelson substituted for Steve Zabriskie as a broadcaster on 60 Mets games covered by WWOR-TV.

[36][37][38] In 2000, Harrelson became part-owner of the Long Island Ducks, a newly formed independent league baseball team.

[2] Harrelson was inducted into the Suffolk Sports Hall of Fame on Long Island in the baseball category in 1992.