Stevenson was educated at St Faughnans College, Rosscarbery and at Queen's College, Cork, qualifying M.B., B.Ch., B.O.A. R.U.I. in 1896. He also took the D.T.M. & H (Cambridge) in 1907 and qualified M.D. (Honours, Gold Medal) in 1913. During this time, on 28 July 1897, he took up a commission in the Army Medical Services at the rank of Surgeon-Lieutenant. In 1898 he took part in the Nile expedition, then was Seconded for service with the Egyptian Army from 7th January 1899 to 6th January 1909. During this time he was in Soudan from 1900 to 1902 and again in 1904, and on 28th July 1900, he gained promotion to the rank of Captain. In 1907 he was awarded the Ottoman Imperial Order of Osmanieh. On 28th January 1909 he gained further promotion to the rank of Major, and from 1909 to 1913 was in charge the Vaccine Department at the Royal Army College. Also in 1913 he won the Parkes’ Memorial Prize and Medal for a paper on “The Causation and Prevention of Enteric Fever on Military Service”. He was Assistant Professor of Pathology at the Royal Army Medical College from 22nd August 1913 to 31st January 1914, after which he was appointed Professor, a position he held until the outbreak of war. When war broke out he was mobilized with Director of Medical Services. [D.M.S.] Surgeon-General Woodhouse as one of his staff at General Headquarters (G.H.Q). Although he was one of the G.H.Q. staff, his duties were not administrative but he was in medical charge of the staff and headquarters' troops. He sailed from Southampton to Havre, entering the war in France on 9th August 1914. With the confusion at the beginning of the war there was a lack of co-ordination between the medical services in the field and those on the lines of communication. As a consequence, he, with the assistance of the mayor of Le Cataeu, organised two hospitals to treat the sick finding their way to headquarters. By the time of The Advance of the Aisne, many medical officers had been killed, wounded or captured, and as a consequence a re-organisation of the staff took place. Neither the 1st or the IIIrd Corps had an administrative medical officer on their headquarters staff during the advance, so Stevenson assisted the D.D.M.S. at G.H.Q. On 24th October his role was acknowledged and he was appointed as D.A.D.M.S. On 1st March 1915 he gained promotion to the rank of Lieutenant Colonel. He was also appointed Companion Order of St Michael & St George during 1915. At the end of August/beginning of September 1916, he took part in a special mission to visit the whole of the French front, to observe the French medical services at work in the French army zones. Also in 1916, due to the development of gas warfare, a directorate of gas services was formed. He was appointed an assistant director of gas services for defensive gas work in March, however, by July the following year the appointment no longer existed. On 3rd June 1917 he was promoted to Brevet Colonel, and on 20th July 1917 he was appointed A.D.M.S., 16th Division. In October/November 1917, due to the Italian field army being forced to withdraw their positions, a British and a French expeditionary force were sent from France to reinforce them. Stevenson was specially detailed to reconnoiter on the medical and sanitary aspects of a campaign in Italy in anticipation of the move of troops. On 28th October, he became A.D.M.S. for the lines of communication, but shortly after a new line of communication was established and a base for the Force was formed, with him being appointed A.D.M.S. of the Base. Before the arrival of a D.D.M.S. on the lines of communication however, and up to the 24th December 1917, he acted both as a D.D.M.S. of the lines of communication and as A.D.M.S. of the Base. He held this appointment until April 1918. In July 1919 he took up his role of Professor of Pathology, R.A.M. College again. He retired from the Army in 1921. He was the son of William Jackson Cummins and Letitia Stevenson; and the husband of Eleanor Fenton Hall, daughter of Robert Constable Hall and Geraldine Maria Aylme, on 28th July 1906.