The son of Thomas E. Headlam, Eustace Slade Headlam was a law student living in Hobart when he enlisted in the 3rd Light Horse Regiment, AIF, on 12 April 1915. After he transferred to the Australian Flying Corps, he served as an observer in Egypt with 67 Squadron in early 1917. He completed pilot training in September 1917 and was posted to 1 Squadron on 2 February 1918. Flying the Bristol Fighter, he scored five victories by the end of the war. He was discharged from the AIF on 4 March 1919; AIF service number 1070.