In the process, Peterson, 33, surpassed one of his idols, Eric Dickerson, to move into eighth on the all-time rushing list.
The Redskins (7-8) went on to lose 25-16 to the Tennessee Titans. Washington, which has lost five of its past six, would be eliminated from playoff contention with a Philadelphia Eagles win or if the Seattle Seahawks and Minnesota Vikings win Sunday.
"Hopefully I'll be able to appreciate it after Christmas or something," Peterson said of his 1,000-yard season and passing Dickerson. "But for the time being, I play this game to win and to have the opportunity to win a championship. So falling short of having that opportunity is what's heavy on my heart right now."
Peterson entered Saturday's game needing 77 yards to reach 1,000 for the season. He ran for 60 yards in the first half and gained the necessary yards on his first carry in the fourth quarter, a 5-yard gain.
Peterson finished with 119 yards on 26 carries, putting him at 1,042 for the season.
He is 2 months older than Frank Gore was when he accomplished the feat two years ago, making Peterson the oldest player to rush for 1,000 yards in a season since former Redskins running back John Riggins did so in 1984.
It's the eighth time Peterson has rushed for 1,000 yards in his career. He said recently that topping that number was "easy" for him. However, he hadn't reached the mark since 2015.
He had 601 combined yards the past two seasons, enduring whispers of his career being over.
Peterson did not sign with the Redskins until several days before the third preseason game in August.
He accomplished the 1,000-yard feat behind a Redskins offensive line that has been ravaged by injuries. The Redskins have started at least five pairs of guards this season; starters Shawn Lauvao and Brandon Scherff both went out for the season in a Week 8 loss to Atlanta.
Since then, it has been a struggle for the run game. Peterson rushed for 90 or more yards five times in the first eight games; he topped 52 yards only twice since then.