Born in London, Tallents was educated at Harrow and Balliol. He began his career as a civil servant at the Board of Trade in 1909 before being transferred to assist William Beveridge and Hubert Llewellyn Smith in establishing labour exchanges. He was commissioned in August 1914 and served as an officer in the Irish Guards in World War I until severely wounded at Festubert. He then worked at the Ministry of Munitions, transferring in 1916 to the Ministry of Food.