He attended the University of Washington (UW), receiving a Bachelor of Arts in political science and graduated as a member of the Phi Kappa Sigma fraternity.

[4] He served as chair of the 34th District Democrats organization and worked as an aide for King County Council member Greg Nickels.

He left the state senate in 2002 after being appointed to the King County Council to replace Nickels, who had been elected Seattle mayor.

Constantine announced his candidacy for King County Executive on February 16, 2009, to replace Ron Sims who was appointed the United States Deputy Secretary of Housing and Urban Development.

[10][11][12] Constantine received press attention for stressing the conservative affiliations of Hutchison, pointing to her involvement with the Discovery Institute and contributions to Republican candidates such as President Bush in 2004 and Mike Huckabee in 2008.

Two-term incumbent Jay Inslee was constitutionally eligible for to run for a third term but had opted to mount a campaign for president of the United States in the 2020 election instead, leaving the governor's office open.

[27] Facing poor polling numbers, Inslee decided to suspend his presidential campaign on August 21 and announced the next day he would indeed seek a third term as governor.

[29][30] In October 2012, Constantine signed legislation supporting a proposed new arena designed to woo an NBA team back to Seattle.

[31] In April 2013, he supported the proposed arena’s lead investor in his effort to purchase the Sacramento Kings and move them to Seattle as the SuperSonics.

[34] Constantine was the subject of a 2018 whistleblower complaint alleging he pressured a county director to award a contract to a company owned by one of his friends.

[39][40] In August 2022, Constantine’s director of communications released a statement implying South King County mayors concerned about crime were “clinging to obsolete practices and mindsets that no longer keep our communities safe.”[41] In October 2022, Constantine cancelled an expansion of a homeless shelter following protests by people with a stake in Seattle’s Chinatown International District.

[43] In January 2024, Constantine announced King County’s youth detention facility would not close under the timeline he previously proposed.