McEwen was born in Chiltern, Victoria. He was orphaned at the age of seven and raised by his grandmother, initially in Wangaratta and then in Dandenong. McEwen left school when he was 13 and joined the Australian Army at the age of 18, but the war ended before his unit was shipped out. He was nonetheless eligible for a soldier settlement scheme, and selected a property at Stanhope. He established a dairy farm, but later bought a larger property and farmed beef cattle.