William Letwin gained entry to the University of Chicago under a plan to benefit the brightest students from poorer schools. He graduated BA in 1943, and joined the US Army, seeing Second World War active service in the Pacific as an intelligence officer on the staff of Douglas MacArthur. Letwin returned to Chicago as a graduate student, then in 1948 transferred to the London School of Economics for two years with the benefit of a Fulbright scholarship. Graduating PhD from Chicago in 1951, he was a post-doctoral fellow in the Economics Department at Chicago, then a research associate in its law school. He was then appointed as an Assistant Professor of Industrial History at the Massachusetts Institute of Technology, and later became an Associate Professor of Economic History there. In 1966 he returned to the London School of Economics as a reader in political science and was promoted to Professor of Political Science, going on to chair the political science department.