Arnold Frederick Sillem was born in Wandsworth in 1865.
He began his army career as a Lieutenant with the 5th Battalion, The Rifle Brigade (The Prince Consort's Own) and on the 4th May 1887 he transferred to The Queen's (Royal West Surrey Regiment) as a 2/Lieut.
On the 12th Frebruary 1891 he was seconded for service with the Egyptian Army. On his return to England, in 1894 Arnold Frederick Sillem was appointed Adjutant of the The Queen's (Royal West Surrey Regiment) a post he held until 12th March 1898.
As a Captain he was appointed to be Assistant Provost-Marshal on the 7th May 1900, with the grade as Deputy Assistant Adjutant General.
He saw service with the 2nd Battalion Queen's (Royal West Surrey Regiment) during the Boer War 1900-1902 and the relief of Ladysmith and was mentioned in despatches in 1901. In recognition of his service during the Boer War he was given the rank of Brevet Major on the 27th September 1901.
On the 13th June 1903 he was appointed Brigade Major of the 13th Brigade, Irish Command (7th Division, 3rd Army Corps), a post he held until 1st November 1906. There then followed a period at Staff College in 1907.
From 1908 to 1912 he was Commander of a Company of Gentlemen Cadets at the Military College, Sandhurst.
On the 17th February 1912 he was appointed to command the 4th Battalion Worcestershire Regiment, with the rank of Lieutenant-Colonel, a post he held until 1914.
On the 20th March 1914 he was appointed Assistant Commandant at the Royal Military College, Sandhurst.
During the First World War in 1915 he was appointed Assistant Adjutant and Quartermaster General on the 26th February 1915 as part of the 3rd Division in the British Expeditionary Force (BEF).