McNabb, who got a rousing ovation from the Philadelphia fans recognizing his 11 years as the Eagles' signal-caller, mostly struggled in the passing department but ran five times for 39 yards in his first game back since being traded in the off-season.
One of his scrambles was an 18-yarder that gave the Redskins a crucial first down in the fourth quarter and all but sealed the win for Washington (2-2).
McNabb finished just 8-of-19 for 125 yards with a touchdown and an interception.
Eagles starter Michael Vick exited the game with a rib injury late in the second quarter and was replaced by Week 1 starter Kevin Kolb, who went 21-of-34 passing for 183 yards and a touchdown for Philadelphia (2-2).
Philadelphia was hampered by penalties, getting whistled eight times for a total of 80 yards.