"I don't want couples to wait a minute longer than legally necessary to get the rights that voters have recognized," Constantine said in a news release announcing the extended hours.
Constantine says he'll be waiting to sign the first license, making King County the first jurisdiction in the nation to issue marriage licenses to same-sex couples, based on voter approval.
Voters in Washington, Maryland and Maine all approved gay marriage initiatives last week, but laws in Maryland and Maine will not take effect until January.
Once they have their license in hand, all couples in Washington still have to wait three days for a marriage ceremony.
King County Marriage License Information