When the Second World War broke out, Robson initially enlisted as a lieutenant in the Militia from 29 September 1939. On 25 December, Robson was granted a leave of absence from the NSW Parliament, with his father dealing with his affairs, and enlisted in the Second Australian Imperial Force on 29 December. Embarking for Scotland with the 2nd/5th Field Regiment, Royal Australian Artillery, in May 1940, he disembarked at Gourock, Scotland on 18 June 1940. Transferring to the 2/31st Battalion in the 7th Division, and was trained in the AIF School. On 3 March 1941, he left Britain, arriving in the Middle East on 9 March. On 9 May 1941, he was appointed to captain.