Westmoreland County is a county located in the U.S. state of Pennsylvania. At the 2010 census, the population was 365,169. The county seat is Greensburg. Formed from, successively, Lancaster, Northumberland, and later Bedford Counties, Westmoreland County was founded on February 26, 1773, and was the first county in the colony of Pennsylvania whose entire territorial boundary was located west of the Allegheny Mountains. Westmoreland County originally included the present-day counties of Fayette, Washington, Greene, and parts of Beaver, Allegheny, Indiana, and Armstrong counties. It is named after Westmorland, a historic county of England.