[3] Although undrafted, Reardon quickly made his way through the Mets' farm system, compiling a 30–9 record with a 2.68 ERA and 6 saves in 74 games combined between Lynchburg, Jackson in 1978, and Tidewater in 1979.

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.

During one bad streak in 1983, Montreal players' wives were invited to appear on the field, and the Expos' fans at Olympic Stadium booed his wife.

During his time in Montreal, Reardon was named to the All-Star team twice (1985–1986), and led the majors in saves during the 1985 season, earning the Rolaids Relief Man of the Year Award.

Although his performance dropped as compared to his previous five seasons, the trade would immediately pay dividends for the team as he would help the Twins win the 1987 World Series.

Later that season, he was traded to the Atlanta Braves for starting pitcher Nate Minchey and minor league outfielder Sean Ross.

[1][7] After the end of his career, Reardon retired to Palm Beach Gardens, Florida at PGA National Golf Club[8] with his wife, Phebe, and three children, Jay, Shane, and Kristi, turning down offers to coach in order to spend time with his family.

By this time, he was taking more than a dozen separate prescriptions, one of which was Levaquin, which is known to have side effects such as paranoia, anxiety and depression—further exacerbating his pre-existing conditions.