Morgan County was named after Daniel Morgan. Daniel Morgan [1736-1802] was an early soldier in the American Colonies, during Pontiac's War [1764] and Dunmore's War [1774].

The area was originally part of Madison County, which was formed in 1812, before Illinois became a state.

The present-day counties of Cass and Scott were part of Morgan County until 1837 and 1839, respectively. The boundary line with Cass County was originally further north than the current boundary which was established in 1845.

Morgan County measures 24 x 24 miles or 576 square miles wide. The county seat is Jacksonville, which is named after General Andrew Jackson.