McMahon was one of the best and most prolific relief pitchers in the 1950s and 1960s. He spun pitches for three teams in the National League and four teams in the American League over an 18-year career. He appeared in 874 games and started only twice, losing both.
He pitched so well for so long that baseball didn’t want to see him go. He retired as a player after the ’72 season with San Francisco and stayed on in ’73 as their pitching coach. When a slew of injuries gutted the SF bullpen in mid-season, the team activated McMahon. In his first appearance he entered a game against the Braves and retired Hank Aaron to end a threat. He got six outs and the save and added five more while pitching to the tune of a 1.48 ERA at the age of 43. He had kept himself in great shape by tossing batting practice as a coach. The next season at the age of 44 he rolled in for emergency duty again, tossing 11 2/3 innings in nine games for the G-Men in May and June.
He won a World Series title in both leagues: with the Milwaukee Braves in 1957 and the Detroit Tigers in 1968.