Montgomery County, locally also referred to as Montco, is the third most populous county in Pennsylvania (after Philadelphia and Allegheny counties). The county has a total area of 487 square miles and the county seat is Norristown. The county was created on September 10, 1784, out of land originally part of Philadelphia County. It is believed to have been named either for Richard Montgomery, an American Revolutionary War general killed in 1775 while attempting to capture Quebec City, Canada, or for the Welsh county of Montgomeryshire (which was named after one of William the Conqueror's main counselors, Roger de Montgomerie), as it was part of the Welsh Tract, an area of Pennsylvania settled by Quakers from Wales. Climate in Montgomery County, PA.