The Wizards came back from a 17-point deficit to stay a half game in front of Philadelphia for fifth place in the Eastern Conference.
Mason made five three-pointers in the fourth period and Antawn Jamison's layup put Washington in front for good with 7:49 to play.
Mason scored 22 points off the bench and Caron Butler had 21 for the Wizards, who played the second game in a five-game road trip.
Kevin Durant led Seattle with 32 points. The SuperSonics had halted an 11-game losing streak Monday with a victory over Portland.