Costes was born in Septfonds, Tarn-et-Garonne. He received a pilot diploma (brevet) on 26 September 1912. During World War I he served in the French Air Service, in MF55 and MF85 Farman squadrons, then in N506, N507 and N531 fighter Nieuport squadrons, on the Balkan front. He scored there 8 victories (6 confirmed), the first in April 1917, the rest in January–September 1918. He ended the war as a 2nd Lieutenant.