Just over one mile long, Maryland Route 509 is found entirely in Port Republic, Calvert County, and was constructed from east of MD 765 in the early 1930's, and it was extended west to MD two and MD four in the early 80's
The entire route is in Port Republic, Calvert County. MD 509 was constructed east from MD 2 (now MD 765) to the bay by 1933. The MD 2/MD 4 divided highway was constructed around 1981; the western extension of MD 509 was built by 1985. Known for most of its length as Governor Run Road, the state highway runs 1.14 miles (1.83 km) from MD 2 and MD 4 east to a dead end at the Chesapeake Bay within Port Republic. 
