The two newcomers hit their first homers Thursday for the Washington Nationals, who salvaged the final game of a three-game series by beating Florida, 5-3.
LaRoche snapped a slump with a two-run homer in the 11th. Werth hit his first homer since signing a $126 million, seven-year contract to join the Nationals, but he went headfirst into third because he thought the ball was still in play.
LaRoche was 3 for 23 with one RBI before he hit his first homer of the year off Edward Mujica (1-1). That ended a streak of 12 2/3 scoreless innings for Florida's bullpen in the series.
Despite a .167 average, LaRoche said he has been making good contact.
"I don't know what I'm hitting. It doesn't feel bad at all," he said. "I think that's because I'm hitting some balls hard. I guess the only safe place to hit it right now is in the seats, it feels like."
The Marlins gave up two unearned runs, struck out 12 times and went 1 for 11 with runners in scoring position.
"We had chances to score more runs but we didn't come through," manager Edwin Rodriguez said.
Washington snapped a streak of seven consecutive losses against Florida dating to July 17. The Nationals improved to 12-33 versus the Marlins since mid-2006.
Four Washington relievers combined to pitch six scoreless innings.
"That's what we're capable of," said Tyler Clippard, who went two innings. "There are a lot of good arms down there. That's what you're going to see all year."
Todd Coffey (1-0), who was ejected from Wednesday's game for arguing about ball-strike calls, pitched a perfect 10th. Sean Burnett, extending a streak of 21 consecutive shutout innings since Aug. 26, pitched the 11th for his second save in as many chances.
Ryan Zimmerman reached on an infield single with one out in the 11th, and LaRoche followed with a homer. That gave the Nationals the win even though they went only 1 for 7 with runners in scoring position. They're batting .140 in those situations and have scored 19 runs in six games.
"Things that could go wrong have been going wrong," manager Jim Riggleman said. "Tonight a couple of things fell for us the right way."
Throwing errors by Florida shortstop Hanley Ramirez and catcher John Buck led to a pair of unearned runs for Washington. The Marlins said improving their perennially shaky defense was a priority this season, but they've allowed eight unearned runs in six games.
The misplays forced ace Josh Johnson to throw extra pitches, and he departed after six innings. Johnson gave up three runs, one earned.
"J.J. was throwing a great game," Rodriguez said. "He was outstanding, but we didn't play defense for him."
Johnson retired the first 10 batters before giving up a bizarre hit: Werth slid headfirst into third base with a home run. The hit landed on top of the right-field wall, and Werth was unaware it had been ruled a homer until he reached third.
"I'm sure he'll hear about it," LaRoche said. "I didn't know it was gone. Obviously he didn't either."
The call stood after a replay review, the second in as many nights for the umpire crew. A similar drive by Florida's John Buck was ruled a triple rather than a homer Wednesday, and that call also was upheld.
The Marlins' first two batters scored before John Lannan settled down. He allowed three runs in five-plus innings.
"My goal was to keep the team in the game, and I did that," he said.
Announced attendance was 10,696, but the actual crowd totaled less than 5,000, creating an atmosphere so subdued that plate umpire Jeff Nelson could be heard from the skybox level during an angry exchange in the fifth inning.
"That's enough," Nelson shouted, pointing toward the Washington dugout. "That's enough."
The Nationals groused about ball-strike calls after Wednesday's loss.
RF Mike Stanton, who missed his fifth consecutive start with a strained left hamstring, struck out pinch hitting in the ninth. The Marlins are hoping he'll rejoin the lineup sometime this weekend at Houston. ... Nationals LF Michael Morse, off to a 2-for-18 start, was held out of the lineup and grounded out as a pinch hitter.