Mark McGwire, 58, 1997
McGwire was the last remaining "Bash Brother" in Oakland when he led the AL with 52 homers in 1996. But with free agency looming and the A's in need of rebuilding, his time in Oakland was drawing to a close. He had 11 homers in April, eight in May and 10 in June, giving him 29 through the season's first 83 games, and he added another five by July 16. But with trade talk overshadowing his slugging, he didn't hit another homer in his final 12 games with the A's before being dealt to the Cardinals for a trio of forgettable righties.
Though reunited with former Oakland manager Tony La Russa, McGwire didn’t homer until his eighth game with his new team, but he finished August with 43 homers, and then launched 13 in an 18-game period from Aug. 28-Sept. 19, giving him 54 with eight games to play. With pitchers approaching him cautiously, he couldn’t quite reach Maris; he closed the season by walking five times and homering three times in his final nine plate appearances. His 58 homers remains the highest total for any player with multiple teams in a single season.