The Shock scored the game's final 15 points after Tamicha Jackson gave the Mystics a 60-59 lead with 4:42 remaining.
In its previous game, Detroit rallied from a 23-point deficit to defeat Indiana, 83-79, in the greatest comeback in WNBA history.
Swin Cash scored 18 points and Cheryl Ford added nine and 10 rebounds for the Shock, who posted their third straight win and avenged a 65-63 home defeat to Washington on June 2.
Chamique Holdsclaw recorded 20 points and 11 rebounds for the Mystics, who dropped to 0-4 at home.