The county was named in 1806 for Meriwether Lewis of the Lewis and Clark expedition, which President Jefferson sent out to explore the Northwest in 1804-05. The expedition followed the Missouri River to its source, crossed the mountains, and then followed the Columbia River to the Pacific. Lewis was born in Virginia in 1774, entered the U.S. Army in 1795, became a captain in 1800, served as Jefferson's private secretary from 1801 to 1803, was territorial governor of Louisiana from 1807 to 1809, and died in 1806 and was buried in Tennessee.