Marr was the son of Leonard and Elvina (née Machon) Marr and was born at their family home in Les Canichers, Saint Peter Port, Guernsey.
He was educated at Les Vauxbelets College, Guernsey, and the College of St Mark and St John, London, graduating with honours in economics and political science.
During World War II, he served in the British Army in France, North Africa, Italy and Austria.
He was a schoolmaster at Hackney Downs, London, specialising in his subjects, and simultaneously taking history.
He wrote a fictional trilogy, Two Men and a Woman, set in the island, and was a regular contributor to The Review of the Guernsey Society.