John left Felsted around 1905 to study medicine at Downing College, Cambridge. He ws a keen all-round sportsman, excellent at golf and tennis but excelled as an oarsman, representing his college (see photograph of him in rowing fours there in 1910, on left). He later trained at St. Bartholomew’s Hospital, London and his first position was House Surgeon at Huntingdon County hospital . Although he qualified as a doctor in 1913 he had still to complete his studies at Cambridge when war broke out in August 1914. By the time he had finished all his medical training he was a Member of the Royal College of Surgeons (MRCS) and Licentiate of the Royal College of Physicians (LRCP).
He was appointed a Royal Army Medical Corps sub-lieutenant on 28th September 1914 and was initially attached to the 5th Battalion, South Staffordshire Regiment, his brother’s regiment, and then a Field Ambulance unit before joining the 5th Battalion, Nottinghamshire & Derbyshire Regt. (Sherwood Foresters) as their medical officer.
His only brother, Second Lieut. Allen (aka, Alan) Edward Green, 5th Battalion, South Staffordshire Regiment, aged 19, was killed at the Battle of Loos on 13th October 1915.