Clarke was one of the most influential and charismatic Franco-Irish generals in the French army during the Napoleonic period. He had close links to the Irish Brigade of France. His father served in Dillon's Regiment (where Clarke himself served for a time) and his mother's father and several uncles served in Clare's Regiment. With the outbreak of the French Revolution, Clarke served in the early French Revolutionary Wars in the Army of the Rhine, and by 1793 had been promoted to "général de brigade". In 1795 Clarke was briefly arrested. After his release, Clarke lived in Alsace until Lazare Carnot sent him to Italy to serve as Napoleon Bonaparte's chief topographical officer, until he was sent to Sardinia. After 18 Brumaire, Clarke served as Chief of the Topographical Bureau, State Councillor, state secretary for the army and navy. During the war against Austria in 1805, Clarke was appointed governor of Vienna and during the war against Prussia in 1806 he served as governor of Erfurt and of Berlin.