Maconochie graduated from the Cadet College at Quetta in 1916 and was gazetted to the 12th Cavalry of the Indian Army on the 5th of July. The regiment had been raised at Lahore in 1857 by Captain P.R. Hockin as the 2nd Regiment of Sikh Irregular Cavalry, becoming the 12th Bengal Cavalry in 1901, and simply the 12th Cavalry in 1903. Maconochie was made lieutenant in 1917 and acting captain in 1919 whilst commanding a squadron. During the Great War, he participated in operations in Mesopotamia (Iraq) from September of 1917 to October of 1918. Maconochie was seconded to the Madras Governor’s Body-Guard as adjutant in 1919 and advanced to captain in 1920. The 12th Cavalry was amalgamated in 1921 with the 11th King Edward's Own Lancers (Probyn's Horse) to form the 5th King Edward's Own Probyn's Horse. Maconochie left the service in 1923 and later joined the stock brokerage firm of Croft, Forbes, and Chard as senior partner. Upon the departure of Mr. Chard, the name of the firm was changed to Maconochie & Co.
