Reardon made his Major League debut with the Mets on August 25, 1979, and would stay in the majors for the rest of his career. He pitched for the Mets until midway through the 1981 season, working mostly as a middle reliever, when he was traded to the Montreal Expos along with outfielder Dan Norman for Ellis Valentine. Lacking a reliable closer on the big league staff, the Expos immediately turned Reardon loose in the ninth inning, and he responded by saving 146 games from 1982 to 1986.