McDermott was elected to the Washington House of Representatives in 2000, taking office in January 2001 and serving until 2007. The 34th district – which he has represented both in the House and the Senate – includes the communities of West Seattle, White Center, Burien (part) and the islands of Vashon and Maury. A Democrat, he was re-elected to the House by wide margins in 2002, 2004 and 2006, winning over 80% of the vote in 2006.
Following the resignation of Sen. Erik Poulsen on 5 September 2007, McDermott expressed an interest in being appointed to the 34th district seat in the Washington State Senate. On 13 October 2007, the precinct committee officers (PCOs) of the 34th district recommended McDermott for appointment with 54 of the 65 PCOs voting for him. Two days later, King County Council unanimously appointed him to the seat. He has to run for election to the Senate in 2008 but no candidate has filed against him and, barring a write-in campaign, he will be elected unopposed.
After graduating with a Bachelor of Arts in history and political science from Gonzaga University, he earned a Master of Public Administration from the University of Washington. He went on to work for the Pierce County Prosecuting Attorney and the Seattle School District.
McDermott is a third-generation resident of West Seattle where he lives with his partner, Michael Culpepper. He is one of six openly gay members of the Washington State Legislature, serving alongside Sen. Ed Murray (D–Seattle) and Reps. Jamie Pedersen (D–Seattle), Jim Moeller (D–Vancouver), Dave Upthegrove (D–Des Moines) and Marko Liias (D–Mukilteo).