Westmoreland County is a county located in the U.S. state of Pennsylvania. As of the 2010 census, the population was 365,169. The county seat is Greensburg. Cut from Lancaster, Northumberland, and later Bedford County, Westmoreland County was founded on February 26, 1773 and was the first county in the colony of Pennsylvania in which its 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.