William Hay had a varied and exciting military career during the later years of the Napoleonic Wars, which took him to the Peninsula, to Waterloo, and, after 1815, to Canada. Graduating from the Royal Military College at Marlow, of which he begins his memoirs with a rare account, he was first commissioned into the crack 52nd Light Infantry and served with that regiment in the campaigns of 1810 and 1811. Promotion then took him into the 12th Light Dragoons and, after a spell at home due to illness, he joined his new regiment in the field just as Wellington's army began its retreat from Burgos. Thereafter, Hay served with the 12th for the remainder of the Peninsular War and again during the Waterloo campaign. A well-connected young man, he spent some of his time away from the regiment on staff duties, serving as an aide to Lord Dalhousie in the Peninsula and later to the Duke of Richmond during the latter's tenure as Governor General of British North America.