After the Legion's offer of the services of two squadrons of horsemen was rejected by the New Zealand government following the outbreak of the First World War, Twisleton enlisted in the New Zealand Expeditionary Force. He was assigned to the Otago Mounted Rifles Regiment, with which he landed at Gallipoli nearly a month after the Landing at Anzac Cove on 25 April 1915. A lieutenant, he continued his letter writing habits of the Boer War, regularly describing life in the trenches in correspondence home. As with his previous writing, British officers were often the subject of criticism. He was awarded the Military Cross for his services during the Gallipoli Campaign, which saw him involved in the Battle of Hill 60. He was also mentioned in despatches. He became ill with dysentery and was evacuated from Gallipoli in September 1915.