The loss knocked the Giants (7-7) out of first place in the NFC East.
Dallas (8-6) now leads the division by a game with two to go, including one with Giants on the final weekend. If New York beats the Jets and the Cowboys in its final two games it will win the division. The Cowboys play the Philadelphia Eagles next weekend and might wrap up the division if the Giants fall in their rivalry game with the Jets.
This loss was the fifth in six games for the Giants, who are now in danger of missing the playoffs for the third straight year.
The scenario has been the same each season. New York starts the season quickly and then falters down the stretch, playing some unbelievably bad games with the postseason on the line.
Two years ago, they lost to the Carolina Panthers in their last home game at Giants Stadium
Last season, they went 10-6 and missed the playoffs because they gave away a 21-point lead in a December loss to the Eagles and then were beaten badly in Green Bay late in the season with a playoff berth in their grasp.
This season it has been another collapse after a stunning win over New England left them at 6-2. They lost four in a row and then last week seemed to grab command again by rallying in the fourth quarter to beat the Cowboys in Dallas.
Again, with the playoffs in their grasp, they came out flat against the Redskins, who had already been eliminated from playoff contention.
Even a couple of gift interceptions by Grossman in the first quarter didn't help.
Gano gave Washington the lead on its second series with a 36-yard field goal that was set up in part by a 19-yard pass from Donte' Stallworth on a third and 12 from the Giants 39.
Grossman, who finished 15 of 24 for 185 yards, pushed the lead to 10-0 early in the second quarter with his TD pass to a wide open Moss on third-and-8.
If the touchdown had the fans muttering, there was no hiding their feelings minutes later after Oshiomogho Atogwe intercepted a pass that deflected off the hands of running back D.J. Ware and returned it 26 yards to the Giants 41. Nine plays later, Young scored and the fans let the team know it was sick of being let down.
Manning, who finished 23 of 40 for 257 yards, finally got the Giants on the board just before halftime, setting up Lawrence Tynes for a 40-yard field goal.
Instead of gaining momentum, the Giants gave the points right back. DeAngelo Hall made a one-handed interception on the second offensive play of the third quarter. His 26-yard return set up Gano's 43-yard field goal for a 20-3 lead.
After Tynes missed a 44-yarder on the next series, Gano opened the fourth quarter with a 25-yarder to make it 23-3.
Ahmad Bradshaw scored on a 3-yard run with 33 seconds to play for the Giants' touchdown.
The Giants were their own worst enemy. Hakeem Nicks dropped what might have been a 54-yard scoring play on their second series on a play in which he got behind the defense and had the pass hit off his facemask as he looked back into the sun.
That's the way the day was for the Giants, who had only a few faithful fans in the stadium when they scored their touchdown.
Washington 23, N.Y. Giants 10
Washington 3 14 3 3 23
N.Y. Giants 0 3 0 7 10
Was • FG Gano 36, 9:14.
Was • Moss 20 pass from Grossman (Gano kick), 13:42.
Was • Young 6 run (Gano kick), 7:56.
NYG • FG Tynes 40, :02.
Was • FG Gano 43, 11:49.
Was • FG Gano 25, 14:57.
NYG • Bradshaw 3 run (Tynes kick), :33.
Attendance • 78,861.
First downs 19 22
Total Net Yards 300 324
Rushes-yards 40-123 18-91
Passing 177 233
Punt Returns 1-(-2) 0-0
Kickoff Returns 3-27 4-104
Interceptions Ret. 3-52 2-0
Comp-Att-Int 15-24-2 23-40-3
Sacked-Yards Lost 1-8 3-24
Punts 2-33.0 4-49.0
Fumbles-Lost 1-0 1-0
Penalties-yards 2-48 8-75
Time of Possession 35:00 25:00
Rushing • Washington, Helu 23-53, Royster 10-36, Young 4-14, Armstrong 1-14, Paul 1-7, Grossman 1-(minus 1). N.Y. Giants, Bradshaw 10-58, Jacobs 8-33.
Passing • Washington, Grossman 15-24-2-185. N.Y. Giants, Manning 23-40-3-257.
Receiving • Washington, Gaffney 6-85, Helu 3-16, Moss 2-40, Stallworth 2-35, Paulsen 1-9, Young 1-0. N.Y. Giants, Nicks 5-73, Cruz 5-44, Manningham 3-57, Bradshaw 3-21, Pascoe 2-26, Hynoski 2-13, Ballard 1-15, Barden 1-6, Ware 1-2.
Missed Field Goals • N.Y. Giants, Tynes 44 (WL).