He enlisted into the 11th Battalion, South Wales Borderers, part of the 115th Brigade, 38th (Welsh) Division. Rees survived the fighting at Mametz Wood, and moved with the Division to Ypres. At Ypres, the Battalion were tasked with the capture of the Pilckem Ridge - a heavily fortified German defensive line during the Battle of Passchendaele.
His citation read:
Sergeant Rees, leading his platoon, gradually worked his way round the right flank, by making short rushes, to the rear of the gun position.
At 20 yards from the machine gun, Sergeant Rees rushed forward towards it, shooting one of the crew, and bayoneting the other. He bombed a large concrete emplacement, killing five of the enemy and taking 30 prisoners, including two officers and capturing a machine gun, undamaged.
During the Second World War, he served as a Company Sergeant-Major in the Home Guard. Rees died on 12 March 1967 at Llanelli, Carmarthenshire, Wales.
Rees is remembered on memorials in Havard Chapel, Brecon Cathedral and at Llanelli Town Hall, Carmarthenshire.