Born in Kingstown (now Dún Laoghaire), County Dublin in Ireland. His father was a Protestant and his mother a Catholic. After leaving what is now Ballymena Academy, he served as a consul in various posts in Africa and South America. While in the British consular service, he exposed the ruthless exploitation of the people in the Belgian Congo and in Peru, distinguishing himself in two reports on "human rights abuses"; He was rewarded with promotion to Consul-General and was knighted in 1911.

Soon after, he returned to his homeland and joined in the struggle for Irish independence from Britain, as one of the leaders of the Irish Republican Brotherhood. In 1914 he went to Germany and attempted to induce Irish prisoners of war to form an Irish brigade to take part in a republican rising. He returned to Ireland in a submarine in 1916 (actually to postpone, not to start, the Easter rising). He was accompanied by a Norwegian manservant, doubtless his lover, but also a double-agent.

As soon as he landed, he was arrested for treason by the British for his involvement in the Irish National cause, was degraded, imprisoned in the Tower, and then hanged in the Pentonville prison.

His controversial diaries, revealing his homosexuality, were made available by the British government in 1959. In his diaries Casement reveals himself as a very busy size queen: "I to meet enormous at 9. Will suck and take too." and "Huge tram inspector . . . stiff as sword and thick and long."

His remains were returned to Ireland by the British government in 1965 and are now in Glasnevin cemetery in Dublin.