Born at Clifton, Bristol, the son of Sir Cornelius Kortright, Guy was educated at Newton College in Newton Abbot, Devon before emigrating to Canada with his family. At age twenty, he decided to continue studies with Louis Grier who ran a painting school at St Ives with Julius Olsson.

In 1899 he shared a studio in Paris with two other St Ives art students, Norman Wilkinson and Hugh Percy Heard. He then remained based in St Ives at St Eia Studio until about 1912 before going to settle in Wales. During WWI he served in the Army and became a Captain in the 21st Monmouthshire Regiment.