Lees was born in Hobart, Australia and was educated at Melbourne Grammar School. He moved to London in 1905 and began studies at the Slade School of Fine Art, joining its staff in 1908. He was a member of the New English Art Club from 1911 and a frequent exhibitor at Vanessa Bell’s Friday Club. He was a friend of Augustus John and James Dickson Innes, and spent the period from late 1910 to 1912 with them at a cottage called Nant Ddu in north Wales where they were known as the Arenig school of painters (named for Arenig Fawr, a mountain in Snowdonia). In 1912 Innes and Lees went on another painting trip to Collioure in Southern France. His wife Lyndra was one of Augustus John's former models. He was the only Australian artist represented at the seminal 1913 Armory Show in New York. His artistic career was curtailed by a mental health problem, possibly schizophrenia, which saw him confined to an asylum from 1918 until his death in incarceration in 1931. He is represented in the National Gallery of Australia, Canberra, the National Gallery of Victoria, Melbourne, the Fitzwilliam Gallery, Cambridge, the Glynn Vivian Art Gallery, Swansea. The Hepworth Wakefield, the Ferens Art Gallery, the Government Art Collection and the Tate.