Whilst at college Hennell had begun travelling around the British countryside to work on essays and illustrations of rural landscapes.
Edward Bawden, a fellow artist, encouraged Hennell to write The Witnesses, an account of his mental illness.
[3] He worked for the Pilgrim Trust in 1940, and the Ministry of Information in 1941, producing watercolours of rural crafts and agriculture in Kent, Dorset, Berkshire and Worcestershire.
[6][7][8] After Singapore, Hennell went to Indonesia and was at Surabaya in Java when he was captured by Indonesian nationalist fighters in November 1945 and was presumed to have been killed shortly thereafter.
Hennell's art works centred on the countryside, and in particular hedging, threshing, baling and the clearing of orchards.
A number of his works are held by the Imperial War Museum, the Tate and are also part of the Ministry of Defence art collection.