The reigning Super Bowl champions have won their last seven games.
Manning coolly completed 21-of-34 passes with one interception, and picked Washington's secondary apart while hardly facing a pass rush.
Amani Toomer, Derrick Ward, and Dominik Hixon all caught five passes.
John Carney kicked three field goals, while Brandon Jacobs scored once on 21 carries for 71 yards for the Giants (11-1), who swept the season series against their NFC East foes.
Washington's Jason Campbell went 23-of-38 passes for 232 yards, no touchdowns and one interception.
He added 38 yards on five carries.
Rookie wide receiver Devin Thomas scored on a 29-yard second quarter run, but the NFL's leading rusher, Clinton Portis, was held to 22 yards on 11 carries for the Redskins (7-5), who have lost two of three.