In January of 1941 Frank Walker was asked to deliver unofficial messages from some members of the Japanese government to President Roosevelt. The request came from Father James M. Drought and Bishop James Edward Walsh, two members of the Maryknoll foreign mission society whose work in Japan had brought them into contact with Tadao Wikawa (also called Paul Wikawa), a prominent Japanese banker with close ties to his government. Walker agreed to serve as an intermediary between the United States government, primarily Secretary of State Cordell Hull, and the Japanese government. The subjects of the negotiations were the strained relations between the two countries and possible agreements that might ease the tensions. Roosevelt asked Walker to make unofficial contacts with the Japanese embassy and to reduce to writing whatever proposals the Japanese made. The records from these negotiations include accounts of meetings, correspondence among the participants, memoranda to Roosevelt and Cordell Hull, chronologies of events, position papers, texts of cables, and newspaper clippings on Japanese- American relations. Most of the contacts were by telephone or personal interview and few written communications were exchanged. Also included in this series are records generated after World War II as historians and Congress investigated the events leading up to the attack on Pearl Harbor. Included in this category are correspondence, excerpts from books on the topic, accounts of the events by various participants, and testimony and reports from the 1946 congressional investigation into the Pearl Harbor attack. Walker's own account of the negotiations and related events can be found in the Memoirs Series (boxes 122-125).

In 1946 Frank Walker accompanied Secretary of State James Byrnes, Eleanor Roosevelt, and Edward R. Stettinius, Jr. to London as a member of the United States delegation to the first session of the United Nations (U.N.) following the adoption of the group's charter at the San Francisco Conference in 1945. Walker's records from these meetings include correspondence, his diary for December 1945 to February 1946, reports of the United States delegation, memoranda reporting meetings with other delegations, U.N. General Assembly resolutions and amendments, lists of U.N. committees and minutes from their meetings, a digest of world press comments on the U.N., and press releases. Correspondents include other members of the United States delegation, organizations and individuals interested in world cooperation, delegates from other countries, and U.N. officials. Walker's work with the United Nations Organization and the U.N. delegation was his last act of public service before returning to his family business and personal interests. Newspaper clippings concerning United Nations activities of this period can be found in the Scrapbook Series.