Monongahela, colloquially called "Mon City," is a Third Class City in Washington County, Pennsylvania, United States (Zip Code 15063) and is part of the Pittsburgh Metro Area, located approximately 17 miles (27 km) south of the city proper. The population was 4,761 at the 2000 census. In 1940, the population of Monongahela was 9,823, in 1950, 10,387, in 1960, 12,948, in 1970, 11,726, in 1980, 8,590, and in 1990.
One of only two cities in Washington County, and the second smallest city in Pennsylvania (after Parker, Pennsylvania), Monongahela sits at the intersection of Pennsylvania state routes 136, 88, and 837, all of which constitute the city's Main Street.
Monongahela is located at 40°12′2″N 79°55′42″W / 40.20056°N 79.92833°W / 40.20056; -79.92833 (40.200462, -79.928394).