Buffalo quarterback E.J. Manuel drove the Bills from their own 34 to a first-and-goal at the Washington seven inside the final minute, but a touchdown pass to Greg Little was nullified by an offensive pass interference penalty on Little.
After another penalty and a sack, Manuel was intercepted on the game's final play.
Manuel finished 21-of-39 for 221 yards.
In his first -- and most likely only -- substantial action of the preseason, Cousins played the entire first half, completing 12-of-23 passes for 188 yards against Buffalo backups. He was intercepted on the Redskins opening drive.
Rookie running back Jonathan Williams pulled the Bills to within 21-16 early in the third quarter with a 37-yard touchdown run. He finished with 42 yard on 11 carries.
On the last play of the third quarter, he was tackled and hit the ground hard, staying down as the trainers came out. He eventually was helped off the field and left the game with a rib injury.
Quarterback Tyrod Taylor played the first two series for Buffalo, completing 2-of-5 passes for 11 yards.
With Redskins running back Matt Jones out for the remainder of the preseason, rookie Robert Kelly carried 12 times for 51 yards. Fellow rookie Keith Marshall carried once for three yards before leaving with an elbow sprain in the first quarter.
Cousins capped a nine-play 69-yard drive -- aided by four Buffalo penalties -- when he hit Reed for a 20-yard touchdown with 2:04 left in the half.
Then, after a Bills fumble, he found Pierre Garcon in the back of end zone from nine yards out.