High Quality Content by WIKIPEDIA articles! Grays Harbor County is a county in the state of Washington, in the United States of America. As of 2010, the population was 72,797. The county seat is at Montesano, and its largest city is Aberdeen. The county is named after a large estuarine bay near its southwestern corner. On May 7, 1792, Boston fur trader Robert Gray crossed the bar into the bay he called Bullfinch Harbor, but which later cartographers would label Chehalis Bay, and then Grays Harbor.