Mediocre catcher overall, but managed for over 50 years- had some of the worst clubs ever, but also some of the greatest- an obvious choice.

I might stand corrected here, but I believe that Cornelius Mcgillicuddy is the only manager or player elected to Cooperstown while their career was still active. Mack retired in 1950 after managing for half a century, with more wins losses and seasons under his belt than anyone else in history. Since he owned the team for most of that period, he had no risk of getting fired. Mack managed in street clothes, eschewing a uniform, and had a gentlemanly manner that belied a pretty ruthless competive spirit. I wouldn't put him among the very greatest of managers, but he is very good.