Frank Thomas had one of the greatest offensive starts to his career in Major League history. Up to age 30, he was basically comparable to Jimmie Foxx and Albert Pujols. He fell off in his 30s, mostly do to injuries, and was a poor fielder who spent a lot of time at DH. But his offensive production early in his career was astounding.
After a short cup of coffee in 1990, Thomas was the White Sox full time first baseman in 1991. That year, he hit .318, led the league with 138 walks and led the league in OBP and OPS, finishing third in MVP voting. He also led the league in OBP, BB and OPS the next year.
In 1993, Thomas hit .317, walked 112 times, hit 41 homers and drove in 128 runs, earning his first MVP award. In 1994, he hit .353 with 38 homers and 109 walks, leading the league in OBP, SLG and OPS. The season ended due to the strike after 113 games, and Thomas was awarded the MVP.
The next three years, his OPS was well over 1.000, and he hit 35-40 homers each season. In 1997, he won the batting title and finished third in MVP voting. At that point, he was 29. In 2000, he again put up MVP caliber numbers, hitting .328 with 43 homers, 112 BB and 143 RBI. He was second in the league in MVP voting.
The decline really started from there.
In 2001, he played just 20 games. The next year, he played 148, but hit just .252. Thomas had a good 2003 season, with a .390 OBP and 42 homers, but played just 108 games the next two seasons combined. He had a resurgent 2006 season with the Athletics, and finished again among the top five in MVP voting, retiring a couple years later.
His career offensive numbers—.301/.419/.555, 156 OPS+, 521 homers—could put him higher, but he spent part of his career at DH and when he was playing the field, he did so poorly.