In August 1914, aged 16, shortly after the outbreak of World War I, Ponticelli was assigned to the 4th Marching Regiment of the 1st Foreign Regiment of the French Foreign Legion. He had lied about his age to enlist. He rediscovered his older brother, Céleste Ponticelli, who had joined the same regiment. According to Ponticelli, France had done much for him, and serving was his way of showing his gratitude. He served at Soissons in Picardy, northeast France, and at Douaumont, near Verdun. Ponticelli worked at digging burial pits and trenches. In keeping a promise to Céleste to always assist others, he rescued a German and a French soldier who were wounded in the arm and leg, respectively.