Casement was born 1864 near Dublin, Ireland. As a young man he worked and travelled in Africa. In 1892 he became an employee of the Consular service, being appointed HM Consul at Lourenco Marques, Mozambique, and St Paul de Loanda in Portuguese West Africa (now Mozambique) in 1895.

In 1899 he was on special service during the South African War and was awarded the Queen’s South Africa Medal. In July 1903 he began an investigation into forced labour in the Belgian Congo's [later Zaire, then the Democratic Republic of the Congo] rubber industry, which was published in February 1904.

In 1908 he was appointed Consul-General to Rio de Janeiro, and investigated reports of atrocities against Amerindians in the Putumayo River region of Peru. He was knighted in 1911. He resigned from the Consular Service in 1913.

In 1913 Casement became treasurer of the Irish Volunteers, an Irish nationalist organisation. He assisted in gun-running for them in 1914. After the outbreak of the First World War he travelled to Germany to try to secure a German declaration of support for an Independent Ireland. He also tried, with little success, to raise an Irish rebel Brigade from among Irish prisoners of war in Limburg camp. He returned to Ireland in April 1916 landing from a German submarine on the eve of the nationalist Easter Rising. He was arrested after the failure of the Rising and imprisoned in the Tower of London.

He was charged with "High Treason", found guilty, and sentenced to death. He was hanged in Pentonville Jail in August 1916. His remains were transferred to Ireland in 1965.