Mercer Run is a long 1st order tributary to North Fork Tomlinson Run in Hancock County, West Virginia. This is the only stream of this name in the United States.
Mercer Run rises about 4 miles south of Newell, West Virginia, in Hancock County and then flows south to join North Fork Tomlinson Run about 2.5 miles north of New Manchester.
Mercer Run drains of area, receives about 37.9 in/year of precipitation, has a wetness index of 299.58, and is about 63% forested.