The Mets are at .500 at the latest point of the season in team history. The 18-18 record is reached as Tom Seaver blanks the Braves at Atlanta-Fulton County Stadium, 5 – 0.

On May 21, 1969 — The Mets are at .500 at the latest point of the season in team history. The 18-18 record is reached as Tom Seaver blanks the Braves at Atlanta-Fulton County Stadium, 5 – 0.

Source:
Baseball Reference May 21