Thomas was a lead investigator in a failed series of experimental treatments for leukemia and for Graft-versus-host disease at Seattle's Fred Hutchinson Cancer Research Center from 1981 to 1993.

Later, he attended the University of Texas at Austin where he studied chemistry and chemical engineering, graduating with a Bachelor of Arts in 1941 and a Master's degree in 1943.

He did his residency at Peter Bent Brigham Hospital before serving two years in the United States Army as an internist stationed in Germany.

"[5] At Mary Imogene Bassett, he began to study rodents that received lethal doses of radiation who were then saved by an infusion of marrow cells.

At the time, patients who underwent bone marrow transplantation all died from infections or immune reactions that weren't seen in the rodent studies.