Joined the Quintin School in 1903 and then the Art School in 1906, first as a student, then as a pupil teacher and then Assistant Master. In 1913 he gained a Scholarship to the Royal College of Art.
From 1914 he served with the Poly A Company of the Rangers, rising from Rifleman to Q.M. Sergeant, before being commissioned into the 7th Battalion Middlesex Regiment. In March 1918 he was wounded and taken prisoner. His drawings from the POW camp are in the collection of the Imperial War Museum.
In 1915 he married Sybil Mayer, a fellow Poly Art School student.
After the war he re-joined the Art School as Assistant Master, at the invitation of the School Head, Mr Percival Gaskell, and taught continuously until his retirement in 1956. On his retirement he announced that he was returning to the Art school as a part-time student to study Lithography and Wood Carving!
Outside of his teaching work, Tresilian had a very successful career as a children’s book illustrator, including Rudyard Kipling’s Jungle Book and numerous Enid Blyton books.