He became active in efforts to organize the automobile industry and was the president of Chrysler Local 7 when it affiliated with the United Auto Workers (UAW) in 1936.

He was a leader of the 1937 Chrysler sit-down strike and that same year was elected a vice president of the UAW.

A series of bitter internal disputes led to Thomas losing the office of the vice presidency in the following year's election, with most of the leading Communists replaced, in what became known as "the biggest setback of all time for the Communists in the American Labor Movement."

After his defeat in 1947, he was named assistant to Congress of Industrial Organizations (CIO) president Philip Murray.

With the merger of the AFL and CIO in 1955, he served under George Meany until his retirement in 1964 due to ill health.