Marcus received his medical education at Edinburgh University, qualifying L.R.C.P. & S. and L.R.F.P. & S. Due to his medical career he became a specialist in bacteriology. He joined the Army Medical Service on 17th November 1899 at the rank of Lieutenant, then shortly after served in South Africa, gaining the Queen’s South African Medal with 5 Clasps and the King’s South African Medal with 2 clasps. On 17th November 1902 he gained promotion to the rank of Captain. He served in Malta from 1908 to 1913, during which time he gained promotion to the rank of Major on 17th May 1911. In 1913 he served in Scutari. After mobilization at the outbreak of war, he formed No 4 General Hospital, then sailed from Southampton to Rouen with the unit as their Commanding Officer on 22nd August 1914, entering the war in France on the 23rd. On 1st March 1915 he gained promotion to the rank of Lieutenant Colonel. In 1915 he also became the Commanding Officer of No 2 General Hospital, a post he served as until 1919. From 1919 to 1923 he served in India. He qualified for a diploma in public health [D.P.H.] in 1924. On 26th June 1926 gained promotion to the rank of Colonel, and from 1926 to 1928 returned to Malta. From 1928 to 1929 joined the British Army of the Rhine. He retied from the Army with pay on 30th June 1930.