The 16th Battalion arrived in France in June 1916 where they took part in the fighting against the Germans at Pozières and Mouquet Farm in August. McCarthy was appointed company sergeant major in March 1917 and the following month was commissioned as a second lieutenant. The day after receiving his commission, McCarthy was wounded at Bullecourt and evacuated to England where he spent three months in hospital and convalescing. Rejoining his unit on 9 July, he was promoted to lieutenant on 1 November and was awarded the French Croix de guerre two days later. Between February and August 1918 he was posted the 13th Training Battalion at Tidworth, England where he trained troops, before rejoining his battalion in time for the Allied Hundred Days Offensive that began on 8 August.