Katz went to work at University College London, initially under the tutelage of Archibald Vivian Hill. He finished his PhD in 1938 and won a Carnegie Fellowship to study with John Carew Eccles at the Kanematsu Institute of Sydney Medical School. During this time, both he and Eccles gave research lectures at the University of Sydney. He obtained British nationality in 1941 and joined the Royal Australian Air Force in 1942. He spent the war in the Pacific as a radar officer and in 1946 was invited back to UCL as an assistant director by Hill. For three years until 1949, the Katz family lived with Hill and his wife Margaret in the top flat of their house in Highgate.