[4] After two more seasons in the Phillies' farm system, the once deemed "untouchable"[5] Thomas was dealt to the Chicago White Sox along with Dick Ruthven and Alan Bannister in exchange for Jim Kaat and Mike Buskey on December 10, 1975.

The Mariners then placed him on waivers toward the end of spring training, then worked out a trade to the Houston Astros for infielder Larry Milbourne.

[7] Thomas was converted to a relief pitcher in 1977, and went 11-6 with a 3.16 ERA and six saves for the triple A Charleston Charlies to earn a call up to Houston that September.

The only two MLB teams Thomas faced in 1977 were the last place Atlanta Braves, and the eventual National League champion Los Angeles Dodgers.

[11][12] After one more relief appearance, he made an emergency start against the San Diego Padres, and gave up five runs in four innings for his first career loss.

He earned a spot in the Cardinals' bullpen in Spring 1980, but got off to a horrible start to the regular season (11.57 ERA & 1 blown save in April).

[16] On May 30, after returning to the bullpen, he struck Montreal Expos outfielder Ellis Valentine in the face with a pitch, shattering his cheekbone.

[17] He remained in the Cards' bullpen another month until he was optioned down to Springfield to make room for Silvio Martinez's return from the disabled list.

[19] He spent his first Spring with his new club in the majors, but was shipped to the triple A Salt Lake City Gulls just as the 1982 season was set to begin.

[22] Despite having missed almost two months of the season, he pitched a team high 93.2 innings in relief, compiling a 7-0 record with a 3.36 ERA and seventy strikeouts.

As a result, Thomas earned the distinction of having compiled the most wins without a loss ever on a team with a losing record when he went 7-0 for the 1985 Seattle Mariners (74-88).

[30] Thomas also pitched for the St. Lucie Legends and the Sun City Rays of the Senior Professional Baseball Association from 1989–1990, until the league folded in the 1990 midseason.