after his graduation from royal military college, sandhurst, arthur fremantle was commissioned into the british army in 1852, serving firstly as an ensign in the 70th foot, before being transferred to the 52nd foot almost immediately thereafter. the following year, fremantle became ensign and lieutenant in the coldstream guards, and continued to receive promotions until, in 1860, at the age of 25, he held the rank of captain of his regiment and lieutenant colonel in the army.