Werth doubled to lead off the fourth inning and later scored, starting a three-run inning that gave the Nationals the lead for good. His solo home run in the fifth gave Washington a 5-1 lead.
Werth signed a $126 million, seven-year contract with the Nationals this offseason. Phillies fans made up a large percentage of the sparse crowd at Nationals Park, and they booed him each time he came to bat - or fielded a fly ball in right field.
Livan Hernandez (1-1) allowed one run on seven hits, struck out six and didn't walk a batter in 6 2-3 innings. He also picked up a sacrifice-bunt RBI as the Nationals won for the fourth time in five games.
Sean Burnett got the final five outs for his third save.
Philadelphia's Ryan Howard led off the second inning with a homer to left, his third of the season.
Joe Blanton (0-1) worked six innings, giving up five runs on seven hits for the Phillies. In two starts this season, Blanton has allowed 12 runs in 10 1-3 innings for a 10.45 ERA.
The Nationals tied the game 1-all in the bottom of the second on Danny Espinosa's sacrifice fly, and grabbed the lead in the fourth. Wilson Ramos doubled home Werth, and Washington loaded the bases on Laynce Nix's single and a walk to Danny Espinosa. Jerry Hairston Jr. singled home Ramos, and Hernandez followed with a surprise bases-loaded sacrifice bunt to score Nix.
Washington increased the lead to 6-1 in the seventh when reliever Antonio Bastardo issued a bases-loaded walk to Ramos.
Philadelphia loaded the bases with one out in both the seventh and eighth innings. In the seventh, Hernandez struck out pinch-hitter John Mayberry on a 62 mph curveball, and reliever Tyler Clippard struck out Shane Victorino to end the inning.
The Phillies were able to score two in the same scenario an inning later. Jimmy Rollins scored on a fielders-choice groundout by Raul Ibanez and Howard came home on a wild pitch by Burnett.
NOTES: Washington put 3B Ryan Zimmerman on the 15-day DL (strained abdominal muscle) and recalled C Jesus Flores from Triple-A Syracuse before the game. ... Nationals 1B Adam LaRoche (groin) and OF Michael Morse (flu-like symptoms) were held out of the starting lineup. Morse pinch-hit in the seventh inning and finished the game at first base. ... Before the game, Werth chatted with his old teammates in the tunnel behind the Phillies' dugout, near the indoor practice batting cages. "He looked the same to me," Philadelphia manager Charlie Manuel said. "Looked wealthier. I told him he had heavier pockets." ... Bastardo's walk to Ramos in the seventh marked the first run allowed by the Phillies' bullpen after 20 straight scoreless innings. ... Burnett gave up an RBI single to Placido Polanco in the ninth, ending a 22 2-3 innings scoreless streak, the longest for any Nationals reliever since the team moved to Washington in 2005.