It was an especially painful loss for the Eagles, who saw quarterback Jalen Hurts depart in the first quarter with a concussion.
Philadelphia failed to clinch the NFC East division title, and fell out of a tie for the best record in the NFC, still shared by the Detroit Lions and Minnesota Vikings who both won on Sunday to improve to 13-2.
The Commanders shook off five turnovers to win, piling up 22 points in the fourth quarter.
Daniels, the second overall pick in this year's draft, found Jamison Crowder on a nine-yard touchdown pass with six seconds left to secure what had seemed an unlikely win.
"The game's never over until the clock hits zero," Daniels said after a win that kept the Commanders' playoff hopes alive while eliminating Dallas and San Francisco from contention.
"It feels awesome," Daniels said. "Everybody can enjoy their Christmas now."
The Eagles, who hadn't lost since September, were without Hurts for most of the game after his head hit the turf when he was tackled by Frankie Luvu at the end of a 13-yard run.
Kenny Pickett replaced him and threw for 143 yards with a touchdown and Saquon Barkley ran for 150 yards and two touchdowns, but the Eagles dropped to 12-3.
Meanwhile NFC North rivals Detroit and Minnesota both improved to 13-2.
The Lions cruised past the Bears 34-17 in Chicago to set a franchise record with their 13th win of the season while the Vikings held off the Seattle Seahawks 27-24.
Stung by a loss to Buffalo that snapped their 11-game winning streak, the Lions jumped to a 20-point lead on the way to setting a single-season franchise record for points scored with 493 so far.
Lions quarterback Jared Goff threw for 336 yards and three touchdowns, including an 82-yarder to Jameson Williams, who had a career-high 143 receiving yards on five catches.
In Seattle, Minnesota quarterback Sam Darnold hit Justin Jefferson with his third touchdown pass of the game with 3:51 to play and the Vikings escaped with their eighth straight win.
Less than two minutes earlier the Seahawks had taken their first lead of the game when quarterback Geno Smith connected with AJ Barner on a four-yard scoring pass.
After Seattle kicker Jason Myers missed a 60-yard field goal attempt with less than two minutes to play, the Seahawks last hope was extinguished when Theo Jackson intercepted a Smith pass in the final minute.
Bills hold off Pats
The Buffalo Bills remained in the hunt for the top seed and home field advantage in the AFC, rallying from a 14-0 deficit to beat the New England Patriots 24-21.
Bills quarterback Josh Allen had a lackluster day, completing 16 of 29 passes for 154 yards and saying later he dealt with numbness in his hand after taking a blow to his elbow.
"It wasn't our best effort -- something we'll learn from," said Allen, whose Bills trail the Kansas City Chiefs in the race for the AFC No. 1 seed.
The Los Angeles Rams remained in control of the NFC West division with a 19-9 victory over the New York Jets.
In Atlanta, rookie quarterback Michael Penix Jr. made his first career start in place of struggling Kirk Cousins and threw for 202 yards in a 34-7 victory over the New York Giants.
The win owed plenty to Atlanta's defense, with safety Jessie Bates and outside linebacker Matthew Judon returning interceptions of Giants quarterback Drew Lock for touchdowns.
Cincinnati quarterback Joe Burrow threw for three touchdowns as the Bengals kept their playoff hopes alive with a 24-6 victory over the Cleveland Browns.
The Miami Dolphins also kept their playoff bid going with a 29-17 victory over the San Francisco 49ers.
