He entered the army on 27 February 1778, just before his 16th birthday, as an ensign in the 1st Foot Guards, a regiment today known as the Grenadier Guards. According to Ambrose Vanderpoel: "Asgill insisted upon entering the army contrary to the wishes of his parents. His father offered to give him a house and £3000 per year 2020 worth £480,948 p.a. if he would adopt some other profession." Asgill became lieutenant in the same regiment with the rank of captain in February 1781.