Skagit County, Washington (Wikipedia) Skagit County /ˈskædʒɪt/ is a county in the U.S. state of Washington. As of the 2020 census, the population was 129,523. The county seat and largest city is Mount Vernon. The county was formed in 1883 from Whatcom County and is named for the Skagit Indian tribe, which has been indigenous to the area prior to European-American settlement.