But Reed has become the defense's top playmaker in the second half of the season. In the past three games, he has delivered five turnovers (four interceptions and one fumble) and two touchdowns.
The Redskins got back into the game after a couple of Ravens turnovers.
Flacco's interception led to a 43-yard field goal, ending the Ravens' shutout 1:20 into the fourth quarter.
Willis McGahee's fumble led to Antwaan Randle El's 5-yard touchdown catch, ending an impressive streak by the Ravens' defense. It marked the first offensive touchdown scored against the Ravens in 13quarters, a span of 46 possessions.
That put Washington within 17-10 with 11:27 left in the game. But the Ravens answered with a run-dominated drive - 11 straight run plays were called - before Flacco (10-for-21 for 134 yards) connected with Mason for a 28-yard touchdown, sealing another win.
"It's great to have an offense that can finish like that," Reed said. "Knowing we can finish on both sides is huge."
Winners of seven of their past eight games, the Ravens finish their regular season against three teams with a combined record of 22-17 (.564).
The Ravens face the Steelers at home Sunday before traveling to the Dallas Cowboys (8-5). Their regular-season finale is home against the Jacksonville Jaguars (4-9).
"I love where we're at and it's not just because of our record," Mason said. "It's because of the attitude that our guys have. We have bought into what the coaches are preaching. The guys are having fun."