Garran accompanied Hughes and Joseph Cook (then the Minister for the Navy) to the 1917 and 1918 meetings of the Imperial War Cabinet in London, United Kingdom, and was also part of the British Empire delegation to the 1919 Paris Peace Conference in Paris, France. There he was on several of the treaty drafting committees, and contributed to many provisions, notably the portions of the League of Nations Covenant relating to League of Nations mandates. Though focusing mainly on League of Nations matters, Garran and John Latham (the head of Australian Naval Intelligence) had the status of technical advisers to Hughes and Cook, and so could attend the main conference and any of the associated councils.