The Governor Thomas Johnson Memorial Bridge is a 1.37-mile (2.20 km) bridge in Maryland over the lower Patuxent River joining Calvert and St. Mary's counties. The bridge, named for the first governor of Maryland, Thomas Johnson, saw construction start in 1972 and opened to traffic on December 17, 1977.

The Patuxent River is a tributary of the Chesapeake Bay in the state of Maryland. There are three main river drainages for central Maryland: the Potomac River to the west passing through Washington, D.C., the Patapsco River to the northeast passing through Baltimore, and the Patuxent River between the two. The 908-square-mile (2,352 km2) Patuxent watershed had a rapidly growing population of 590,769 in 2000. It is the largest and longest river entirely within Maryland, and its watershed is the largest completely within the state.