Thomas's biography gives an account of his war years. When France fell to the Nazis, he lived in Nice, under the Vichy government, changing his name to Michel Thomas so he could operate in the French Resistance movement more easily. He was arrested several times, and finally sent to Camp des Milles, near Aix-en-Provence. In August 1942, Thomas got released from Les Milles using forged papers and made his way to Lyon, where his duties for the Resistance entailed recruiting Jewish refugees into the organization. In January 1943, he was arrested and interrogated by Klaus Barbie, only being released after convincing the Gestapo officer that he was an apolitical French artist. He would later testify at the 1987 trial of Barbie in Lyon, although the prosecutor "threw doubt" on Thomas' testimony with regard to the "difficulties of identification" after so much time had elapsed.