Prohibited by the bonus rule of the time from being sent to the minor leagues, he appeared in only one big-league game in his first pro season, striking out as a pinch hitter against Ted Abernathy of the Washington Senators on September 11, 1957.

Inserted into the game at third base, he made an error on his only fielding chance, but the miscue and the unearned run it produced were harmless, as Detroit won, 12–2.

But his playing time diminished in 1965, his average sank to .213, and on October 4 he was traded to the Red Sox with second baseman George Smith and a player to be named later (catcher Jackie Moore) for pitcher Bill Monbouquette.

However, Thomas was a utility man and pinch hitter on the 1967 "Impossible Dream" Red Sox, appearing in 65 games, including 43 in the outfield, three at third base and one as a catcher.

He was briefly listed as a coach for the Red Sox in 1969 and 1970 before returning to active status, and finished his career with his hometown Twins after Boston released him, June 28, 1971.