William McDiarmid Allardice was born on 31 Aug 1875, the son of George James Campbell Allardice and Jane Dalrymple Philip. Commissioned 2nd Lieutenant with the 7th Royal Fusiliers on 28 Mar 1900 and 2nd Lieutenant of the 2nd Battalion, Essex Regiment on 21 Apr 1900, Allardice was promoted to Lieutenant on 4 Oct 1901 and invalided home with Captain L.E.G. Oates aboard the S.S. Plassy in 1902, as the Essex Chronicle 13 June 1902 reported: "Lieut. L.E.G. Oates, 5th Dragoons, son of Mrs Oates of Gestingthorpe, is being invalided home on board the steamship Plassy, which is due at Southampton on 21 June. Lieut. W. McD. Allardice and Second Lieut. P.W. Huskisson, 2nd Essex Regiment, are also being invalided home on the same boat."
Seconded for service in the Army Ordnance Department on 22 Apr 1904, Allardice subsequently was aapointed Inspector of Small Arms, Birmingham Area, Ministry of Munitions and awarded the O.B.E. (Military Division) Gazetted 7 June 1918.