The Broncos trailed 21-14 after three quarters, but Manning uncorked three scoring strikes in the fourth and Denver pulled away.
The NFL's leading passer completed 30-for-44 passes for 354 yards, hooking up with Knowshon Moreno, Demaryius Thomas and Joel Dreessen on late TD tosses while overcoming three interceptions and lost a fumble.
The Broncos (7-1) used the triumph to stay within one game of Kansas City (8-0) in the AFC West.
Robert Griffin III managed to make good on 15-of-30 pass attempts for 132 yards, one touchdown and two interceptions for the Redskins (2-5).
Washington head Coach Mike Shanahan lost in his return to Denver, where he led the Broncos for 14 seasons and won two Super Bowls before being fired in 2008.