Dartnell joined the 86th (Royal County Down) Regiment of Foot as an Ensign in 1855 and was promoted to Lieutenant in 1856. During the Indian Mutiny (1857–59) he served with his regiment during the storming of Chanderi in March 1858 and at the storming of Jhansi Fort on 3 April 1858 when he was disabled by wounds received while climbing the ladders to scale the fort's wall. The first man to ascend the ladder, Dartnell was set upon by the mutineers who slashed him with their sabres and shot him. He was recommended for the Victoria Cross but was not awarded it. For his gallantry he was promoted to company commander and was invalided to Britain to recover from his wounds. He was promoted to Captain in 1859 and transferred to the 16th (Bedfordshire) Regiment of Foot.