He joined the Virginia militia in 1794 and helped put down the Whiskey Rebellion. He remained in the militia and advanced to the office of paymaster. He served on the frontier, tromping the woods and trails to give soldiers their pay. This training would come in handy on the expedition west.
Lewis was an explorer not by choice but by default. He was a close personal friend of Thomas Jefferson, who was president at the time of the Louisiana Purchase. Lewis was also, at that time, Jefferson's private secretary.
Lewis was a natural choice to command the Corps of Discovery, the official name of what we now call "The Lewis and Clark Expedition." He had learned how to determine latitude and longitude, as well as how to classify plants and animals.
Incredibly, on the journey west, he discovered 173 new plants and 122 species and subspecies of animals. All of these records were turned over to President Jefferson when Lewis and William Clark returned from their journey.
Lewis survived a skirmish with Native Americans and a friendly-fire shot. When he made it back home, he was quite famous. However, he wanted to return to the West and persuaded Jefferson to appoint him governor of the Louisiana Territory.
He never again found success or fame. He died alone on Oct. 11, 1809, near Nashville.