Phone:(412) 350-5300 Fax:(412) 350-3581 info.co.allegheny.pa.us In southwestern PA; organized Sep 24, 1788 from Westmoreland and Washington counties.Name Origin:For the Allegheny River Area(sq mi)::744.71 (land 730.17; water 14.54)Population per square mile:1692.50 ...