Educated privately and at Gresson's School in Worthing, Harington continued his studies at Cheltenham College. An interest in sports, particularly cricket and swimming, developed during his school years. He was admitted into Sandhurst in 1890 and after graduation two years later was commissioned as a second lieutenant into the 2nd Battalion the King's Liverpool Regiment on 9 January 1892. He was promoted to lieutenant on 4 February 1893, and became its adjutant in 1897. After the outbreak of the Second Boer War in late 1899, Harington was seconded as a railway staff officer in South Africa, and was promoted to captain on 21 March 1900. For his service in the war, Harison was decorated with the Distinguished Service Order (DSO), and he was characterised in a despatch as being "an officer of first-rate ability, business capacity and tact".