Back in 1966, the Red Sox traded once all-star reliever Dick Radatz for McMahon. As it turned out, Radatz was washed up and was out of baseball a couple years later. McMahon was good in 1966 for the Red Sox, but wasn't stellar for them in 1967, essentially being replacement level. In that year, the team that would end up being the "Impossible Dream Team" traded McMahon and minor leaguer Rob Snow for infielder Jerry Adair. To the Sox's credit, they got a lot of value out of Adair, although he'd be dreadful in 1968.
Meanwhile, after being traded to the White Sox, McMahon pitched very well for the rest of 1967 and 1968 for the White Sox. He'd move to the Tigers, where he was adequate, and pitched several more seasons for the San Francisco Giants, including one of his best years in 1970 at age 40. McMahon would still have several seasons left in him, although he was never quite that good again.
So, this was a pretty good trade for the Red Sox, and it's a good card for vintage Red Sox baseball card collectors. Excellent condition copies can be found for $2 or less. While McMahon might not be remembered much today, he was definitely an above-average relief pitcher for a very long time. Not only was he good for the Red Sox for a bit, but he again contributed by acquiring a very useful infielder for the 1967 team. As of 2018, Don McMahon is still with us, too.
Here's to what was a great, and pretty underrated, career!