Brittney Sykes had a career-high 27 points for the Mystics, who finish at second-place New York. Elena Delle Donne had 13 and Shatori Walker-Kimbrough had 11. Natasha Cloud had a career-high and franchise record 15 assists.
The Dream opened the fourth quarter with a 12-0 run for a 73-60 lead, their biggest of the game. Gray started it with a jumper and a 3-pointer.
Walker-Kimbrough and Cloud had consecutive 3s to pull Washington to 75-68 with 5:44 left and Sykes made it 76-75 with 20.3 to go.
Gray and Robinson then made free throws in the final seconds.
The Dream led 42-35 at the half and by as many as 11 in the third quarter but Sykes scored 15 in the third to cut the deficit to 61-60 entering the fourth.