Political adventurer Harman Blennerhassett (October 8, 1764 - February 2, 1831) was born at Hambledon, England, where his wealthy Irish parents were visiting. Blennerhassett's education included London's Westminster School and Trinity College, Dublin, and legal training at the Inns of Court, London, and at Dublin's King's Inns.

After inheriting the family estate in 1792, Blennerhassett embarked upon a fateful chain of decisions, joining in 1793 the secret Society of United Irishmen, which plotted the overthrow of English rule in Ireland, and wedding his niece, Margaret Agnew, in 1794. He sold his property in 1795 before emigrating to America to escape arrest for his subversive activity and to avoid family friction over his marriage.

Landing August 1, 1796, at New York, the Blennerhassetts settled in 1798 on Belpre (later Blennerhassett) Island, in the Ohio River near present Parkersburg. In September 1800, their new home, an enormous Palladian structure, was completed, its rooms filled with imported furnishings. The mansion's beauty, romanticized by an island setting, was enhanced by nine acres of lawns and elaborately landscaped gardens.

By 1805, the Blennerhassetts accepted Aaron Burr's invitation to join his expedition to the Southwest. Many historians now believe this mysterious enterprise was to have been against Spanish-ruled Texas, and not, as Burr's detractors maintained, a plot to separate the West from the United States. President Thomas Jefferson, however, ordered Burr’s and Blennerhassett's arrests late in 1806 on a charge of treason. Harman fled downriver, followed a few days later by his wife and children.

Soon Burr and Blennerhassett were captured and imprisoned in the Virginia State Penitentiary. Burr's subsequent acquittal freed Blennerhassett, who was released on January 4, 1808. He spent the rest of his life futilely attempting to rebuild his fortune. Returning impoverished to England in 1824, he died on Guernsey in the Channel Islands.