Gresley was born in Derbyshire and studied at Derby School of Art. He interrupted his studies when the First World War broke out, joining 1st Battalion The Royal Fusiliers. During the war he fought in the Balkans and in France. For his gallantry in France in late 1918, he was awarded the Distinguished Conduct Medal, a British award second only to the Victoria Cross.

After the war he continued his studies in Nottingham under Arthur Spooner, and became a teacher at Repton School. Though the Gresleys are principally known for their landscapes, Harold was a highly accomplished portrait painter too. He lived at Chellaston, near Derby until he died in 1967.