The Redskins (9-7) had already clinched the NFC East title and a playoff berth.
Washington will host either Minnesota or Green Bay next week in the first round of the NFC playoffs.
Cousins and a number of ‘Skins starters played limited minutes as coach Jay Gruden wanted to keep his team sharp and as injury-free as possible heading into the Super Bowl tournament.
Cousins was 12-for-15 passing for 176 yards and three scores, pushing Washington to a 24-0 second-quarter lead.
Colt McCoy was 7-for-11 passing for 128 yards and a touchdown in mop up duty for the Redskins, which won four straight to finish the regular season.
Dallas (4-12) finished its season on a four-game losing streak and its worst record since 1989.
Kellen Moore threw for 435 yards and three touchdowns, but he was also intercepted twice.
Alfred Morris ran for 100 yards on 19 attempts for Washington.