Henderson embarked for the Middle East with the main body of the NZEF in October 1914. Soon after the ANZAC landings at Gallipoli he saw John Simpson Kirkpatrick using a donkey to carry wounded soldiers, and began to do the same. While it is reported that he began this work after Kirkpatrick's death on 19 May 1915, he was photographed with a donkey carrying a wounded man on 12 May 1915 by Sergeant James Gardiner Jackson. According to Henderson's own account, he continued the work for about six weeks after Kirkpatrick's death.