[1] Its county seat is Coupeville,[2] while its largest city is Oak Harbor.

It originally encompassed what are now Snohomish, Skagit, Whatcom, and San Juan Counties.

As of the 2020 census, there were 86,857 people, 35,917 households, and 24,229 families living in the county.

There were 41,922 housing units at an average density of 201.1 inhabitants per square mile (77.6/km2).

As of the 2010 census, there were 78,506 people, 32,746 households, and 22,156 families living in the county.

SR 20 enters Island County via the Port Townsend-Coupeville (Keystone) ferry route from the West, and departs via the Deception Pass Bridge in the North.

While the north (Oak Harbor) is conservative – Donald Trump received almost 55 percent of the 2016 vote and carried most precincts – most southern and central precincts voted for Hillary Clinton.