The first Australian to qualify for a Royal Aero Club flying certificate, in 1911, Watt joined the French Foreign Legion as a pilot on the outbreak of World War I. He transferred to the Australian Flying Corps (AFC) in 1916, quickly progressing from flight commander with No. 1 Squadron in Egypt to commanding officer of No. 2 Squadron on the Western Front. By February 1918, he had been promoted to lieutenant colonel and taken command of the AFC's 1st Training Wing in England.