He was appointed Recorder of Dudley (1936–38) and Recorder of Oxford (1938–51 & 1956–64). He lectured on private International Law at Oxford (1934–39) and The Hague. In 1939, he was in the United States and volunteered his services to the British Embassy. He was immediately appointed First Secretary and legal advisor. During this period, the United States was still a non-belligerent, and President Roosevelt faced many difficult problems related to the international law of neutrality. Foster played a significant behind-the-scenes role working on neutrality issues related to the Destroyer-for-Bases deal in 1940 and the Lend Lease Act in 1941.