John received his medical education at Guy’s Hospital, qualifying M.R.C.S.,L.R.C.P. in 1895. On 29th January 1896 he took up a commission in the R.A.M.C. at the rank of Lieutenant. From 1897 to 1898 he served on the N.W. Frontier, India. On 29th January 1899 he gained promotion to the rank of Captain. In 1904 he gained a diploma in public health [D.P.H.] and a diploma in tropical medicine [D.T.M.] He gained further promotion to the rank of Major on 29th July 1907.
In August 1914, it was decided that the formation of an expeditionary force against the Cameroons would consist of British and French troops under the command of the Inspector-General of the West African Frontier Force, Brigadier-General C M Dobell, John was appointed, by General Dobell, to be the Director of Medical Services [D.M.S.]. He accompanied the Expeditionary Force to the Cameroons from Sierra Leone, and hurriedly created the medical services of the campaign whilst at sea from personnel picked up as he went down the coast. Before disembarking he had arranged a regimental medical service for each battalion, four sections of a field ambulance, and a medical officer was appointed to oversee convoys for evacuation to the advanced base and from there to a hospital base. He was D.M.S. throughout the campaign in the Cameroons, and wrote a detailed report of the medical services there, which was used in the writing of the official medical history.
On 1st March 1915 he gained promotion to the rank of Lieutenant Colonel. From April 1915, when operations centered at Wum Biagas, an allied force of both French and British began to be formed. John joined the allied force in order to direct the medical arrangements of a comprehensive scheme which had to be considered for the evacuation of sick and wounded, and for establishing medical posts.
Between August to November 1916 he acted as a liaison officer, on behalf of the British medical authorities, appointed to the French Army Headquarters. His work was to be in touch with the French liaison officer who had charge of the general arrangements for the reception and evacuation of Serbian sick and wounded. Also in 1916 he received his C.M.G. in 1916 [Companion Order of St Michael & St George] for an act of bravery in the field, removing wounded under a hot fire and recovering a gun which had been abandoned.
In January 1917 he was attached to railway and engineer staff, which examined the formation of a Mediterranean Line of Communication, or first know as an Eastern Line of Communication, which had been established through Italy and France in connexion with operations in Macedonia. He made a second reconnaissance in May 1917. Shortly after, on 15th May 1917, he was appointed temporary Colonel whilst A.D.M.S. of the Mediterranean line of communication, due to special appointments having to be made for the administration of its medical and sanitary services. In October 1917, he was graded as a D.D.M.S.
In November 1917, when the British force was sent to Italy, he was appointed A.D.M.S. Paris, an area which included the French section of the Mediterranean line of communication and all the parts of France which did not come into any other British administrative area, but which British services were employed. He was still acting as A.D.M.S. in the British Paris administrative area, with the French on the Aisne and the Marne May to August 1918.