[1][a] Returning to government service in 1940, Clayton first Later in World War II, he took on a number of roles in the Roosevelt and Truman administrations.

He served as assistant, and then as deputy Secretary of State for economic affairs from December 1944 to October 1947, where he was primarily concerned with working on the Marshall Plan.

He returned to Houston and private life in late 1947, though he continued to serve the government as a participant and contributor to various international conferences on world trade and other economic issues.

[1] Leaving school at age 13, he became an expert stenographer, which earned him a job as private secretary to Jerome Hall, a Saint Louis cotton merchant.

[1] In 1940, Clayton returned to government service in the Reconstruction Finance Corporation, then moved to the Export-Import Bank, where he worked to procure strategic materials for the United States and to deny them to Nazi Germany.

[4] At the end of 1944, Clayton was named the first Assistant Secretary of State for Economic Affairs, a post that allowed him to promote the free trade policies that he believed in.

[5] Clayton strongly supported American economic aid to rebuild Europe after World War II and had a major role in shaping the Marshall Plan in 1947.

He cautioned that such a move would contradict the recommendations of the Anglo-American Committee and the Morrison–Grady Plan, potentially inciting hostility from the Arab world.

[7][8] In 1948, he returned to his private business in Houston but remained active in efforts to promote free trade and economic cooperation between the United States and its allies during the Cold War.

[1] William Clayton died in Houston, Texas February 8, 1966, after a short illness, and is buried there in Glenwood Cemetery.