Nemacolin was a leader of the Delaware (Lenape) people during the difficult middle decades of the eighteenth century, when his nation was being pressed westward by colonial expansion. He is best known to American history as the namesake of Nemacolin's Path - the trail across the Allegheny Mountains that became the basis for Braddock's Road and eventually the National Road and U.S. Route 40. The trail was the principal route into the Ohio Country in the 1750s and 1760s. By the late 1760s, Nemacolin had moved to the Ohio River and settled on the long island below the mouth of the Little Kanawha that would later carry someone else's name. He died there in 1767. The island was already a meaningful place before any European-American owned it.

Elijah Backus bought the island in 1792, during the wave of land acquisition that followed the American Revolutionary War. For five years, the property was known as Backus Island, a holding in the same speculative real-estate economy that had pulled Rufus Putnam to Marietta and that would eventually fill the Ohio Country with new American settlements. In 1798, Harman Blennerhassett purchased the eastern end of the island from Backus and began building his mansion. The name shift from Backus to Blennerhassett happened within a few years - one of those small acts of colonial naming that erases the original Lenape name and replaces the intermediate American name without explanation.

What happened on Blennerhassett Island between 1805 and 1807 has been argued about ever since. Aaron Burr, former Vice President, visited the island in 1805 and convinced Harman Blennerhassett to back a venture whose actual goals remain unclear: possibly the seizure of Spanish-held territory in what is now the American Southwest, possibly an attempt to provoke war with Spain, possibly the separation of trans-Appalachian states from the Union. Whatever Burr was actually planning, Virginia militia seized the island in December 1806, the conspirators were arrested, and Burr was tried for treason in Richmond in 1807. He was acquitted - the prosecution could not meet the constitutional requirement of two witnesses to an overt act of treason - but the trial set a foundational American precedent for treason prosecutions that remains in force today. The Blennerhassetts, ruined financially and reputationally, never recovered.

The island stretches roughly four miles down the Ohio River, with the Blennerhassett mansion replica and the state historical park occupying its upstream end. The Blennerhassett Island Bridge - carrying U.S. Route 50 between Parkersburg, West Virginia, and Belpre, Ohio - crosses the river just downstream from the island, with its central pier resting on the island itself. The bridge opened in 2008 after more than a decade of planning and construction. The state park is accessed only by sternwheeler from Point Park in Parkersburg, with no road bridge to the island's upstream end. Most of the island remains undeveloped bottomland forest. The river is wide and slow here, with barge traffic working between the locks and dams that step the Ohio down toward the Mississippi.

From the air, Blennerhassett Island appears as a long, narrow, wooded landmass in the middle of the Ohio River, longer than nearly any other island in this stretch of the waterway. The state park's cleared area at the upstream end - mansion, dock, carriage paths - shows as a green opening in the otherwise continuous tree canopy. The Blennerhassett Island Bridge crosses the river immediately to the south, a network tied arch span with its central pier rooted on the island. Parkersburg sits on the West Virginia bank to the east, Belpre on the Ohio bank to the west. The Little Kanawha River joins the Ohio just upstream of the island - the geographic reason this stretch of water was important enough to fight over.