Educated at Sherborne School and the Royal Military College, Sandhurst, Monro was commissioned into the 2nd Regiment of Foot as a second lieutenant on 13 August 1879.

[9] He vacated that appointment in February 1900,[10] as he went to South Africa to serve in the Second Boer War, where he was present at the Battle of Paardeberg in 1900.

[12][13] Promoted to colonel in 1906, the same year he was made a Companion of the Order of the Bath (CB) in the 1906 Birthday Honours,[14] he was succeeded Henry Merrick Lawson as commander of the 13th Infantry Brigade in Dublin on 12 May 1907, with the temporary rank of brigadier general.

[17][18] In the early days of the First World War on 5 August 1914, Monro was deployed to France as General Officer Commanding 2nd Division, which played an important part in the First Battle of Ypres.

[24] In October 1915, the seventh month of the Gallipoli campaign, General Sir Ian Hamilton was dismissed as commander-in-chief (C-in-C) of the Mediterranean Expeditionary Force.

[25] The Allied position had been drastically altered by the entry of Bulgaria into the war and the Central Powers' subsequent swift conquest of Serbia, which opened the railway from Germany to Constantinople for transporting heavy guns and ammunition.

[26] After three days conferring and inspecting the three beachheads, Monro cabled Secretary of State for War Herbert Kitchener to recommend evacuating "the mere fringe of the coast-line" that had been secured.

[28] The War Committee dithered, finally on 7 December agreeing to evacuate two of the bridgeheads (ANZAC Cove and Suvla Bay).

Their reluctance was understandable: Ottoman guns were able to strike the landing zones on all three beachheads, so evacuation casualties were estimated at thirty to forty per cent – Monro requested fifty-six hospital ships.

After further pressure from Monro, the evacuation of the remaining beachhead at Cape Helles was authorized on 28 December with the agreement of the French who had troops there.