Harrelson anchored the Mets’ infield for thirteen seasons, including their 1969 season, and 1973 pennant-winning season. Harrelson was typical of shortstops of his era: good fielder, poor hitter. He had a lifetime batting average of .236 and hit a total of seven homeruns during his fifteen year major league career, but had a lifetime .969 fielding percentage and won a Gold Glove at his position in 1971. He was a Natinal League All-Star in 1970 and received Most Valuable Player consideration despite batting only .243 for the season.

After retiring as a player he got his first managerial job with the NY Mets organization with the Little Falls Mets. He led his Little Falls team to its first and only New York Penn League Championship. He served as an assistant coach for the World Champion 1986 Mets, and as manager of the NY Mets in 1990 and 1991. He was inducted into the New York Mets Hall of Fame in 1982.

He currently resides in Hauppauge, New York, and is co-owner, Senior Vice President for Baseball Operations and first base coach of the Long Island Ducks, an unaffiliated minor Leage baseball team.