[5] Harrington was born in Chichester, the son of Emanuel Thomas Poë and Isabella Jane (née Crowdy), and christened Charles Harington.

[6] Of Anglo-Irish heritage, Harington gained his unusual name repetition as a four-year-old infant, when his father replaced the family's original surname of "Poë" with the maiden name of Charles's grandmother.

Fellow officers from his battalion assigned Harington this nickname while en route to Aden after learning of the conviction of Timothy Charles Harrington, an Irish nationalist and Member of Parliament.

[13] Recalled as adjutant of the 4th King's Regiment, Harington served with the battalion in Ireland for about a year under the command of Lieutenant Colonel Grattan (his future father-in-law).

[16] He was selected for employment at the Staff College, Camberley in 1906 – a decision that had neither been anticipated or sought[17] – and later moved to the War Office and Army Headquarters.

My greatest friends, my brother officers, all my old company except one or two, had gone; the majority had made the supreme sacrifice....[23]The British Expeditionary Force began to reorganise immediately after the Battle of Mons and subsequent retreat to the Marne.

Harington became General Staff Officer, Grade II (GSO2) in III Corps, formed under Lieutenant-General William Pulteney to group the 4th and 6th divisions.

[27] Months later, Harington learnt that Perceval's absence had necessitated his retention, precluding his transfer to the Macedonian front to serve with the British Salonika Army's XII Corps.

[31][35] As General Plumer's chief of staff, on the day before the Battle of Messines, Harington, who had been created a Companion of the Order of the Bath in January 1917,[36] famously said to reporters, Gentlemen, I don’t know whether we are going to make history tomorrow, but at any rate we shall change geography.At dawn the next day, June 7th, the largest man made explosion on earth shook the air as 19 mines filled with explosives detonated in front of the lines of the Second Army, killing approximately 10,000 Germans.

Supporters of Dyer, mostly imperialist conservative "diehards", army officers and Ulster Unionists, opposed any form of disciplinary action being taken against the brigadier, whom they considered a defender of the British Empire.

[40] According to Nigel Collet, in his book Butcher of Amritsar, Harington's schedule afforded him limited opportunity to comprehensively study Dyer's statement detailing his account while he did not "appear to have read any other document on the case".

[40] Under pressure from the Secretary of State for War Winston Churchill,[41] the Army Council recommended that Dyer not receive further employment or promotion but elected not to publicly endorse compulsory retirement.

[42] After relinquishing his position as DCIGS to General Sir Philip Chetwode in 1920, Harington assumed command of the Army of the Black Sea, occupying parts of Turkey and later used to enforce a neutral zone established by the nominal signing of the Treaty of Sèvres during the Greco-Turkish War.

[44] In 1921, the Greek Army in Anatolia initiated an offensive against Mustafa Kemal's forces and maintained their advance with the intent of capturing Angora.