Contributions to neurology by Symonds include a highly accurate description of subarachnoid haemorrhage in 1924, and idiopathic intracranial hypertension (which he termed "otitic hydrocephalus") in 1931.

[1] On the outbreak of the First World War, Symonds left his medical studies and joined the British Army, serving as a despatch rider in the motorcycle section of the Royal Engineers.

He was wounded at the start of the Race to the Sea in September 1914, and was awarded the Médaille militaire, and then returned to his medical studies at Guy's.

[1] He married Janet (née Palmer), daughter of Edward Bagnall Poulton in 1915, they were to have two sons, one of whom was the political satirist Richard Symonds.

[1][3] After his MRCP in 1916 he returned to France with 101 Field Ambulance and medical officer to 1st battalion, the Middlesex Regiment,[1] and was promoted temporary captain on 8 May 1916,[4] He resigned his commission on the grounds of ill-health contracted on active service on 2 February 1919.

[5] Symonds completed his medical studies in 1919, gaining his MB BCh and also his Oxford Master of Arts and Doctor of Medicine.

[1][9] Having been appointed a civilian consultant in neurology to the RAF in 1934, he was commissioned as a group captain on 11 September 1939, just after the outbreak of the Second World War.

[1] As the war progressed, much of his work centred on a condition called "flying stress", with Denis Williams he analysed over 3000 case studies leading to the report Clinical and Statistical Study of Neurosis Precipitated by Flying Duties[11] for which Symonds was awarded the 1949 Raymond F. Longacre award, administered by the U.S. Aerospace Medical Association (AsMA), for scientific contribution to aviation medicine.

[1] In 1954, he gave the Harveian Oration at the Royal College of Physicians on 18 October[17][18] and, in 1956, was elected president on the Association of British Neurologists.