Miami's Tom Koehler was solid in six innings, but he was hurt twice by McLouth, who belted a homer to right field to open the sixth.
Giancarlo Stanton's RBI double was the lone run allowed by Jordan, who scattered five hits in his 86-pitch performance.
Koehler and Jordan are looking to lock down fifth-starter rotation spots for their respective clubs.
Washington won all four Grapefruit League matchups with Miami this spring.
About an hour before game-time, there were questions as to whether the game would be played. Because of rain, the teams were preparing for at least a half-hour delay. Instead, it cleared enough, and the grounds crew was able to get the field ready for first pitch..
The game started with Koehler surrendering three consecutive hits, which led to a run. Denard Span was credited with a single on a hard grounder that went off the glove of first baseman Garrett Jones. But Koehler picked Span off.
Jeff Kobernus singled and scored on McLouth's RBI double down the right-field line.
After McLouth's double, Koehler didn't allow a hit until the sixth, when McLouth belted a home run to lead off the inning, giving the Nationals a two-run advantage.
The Marlins threatened in the first, but Jordan worked out of the jam. Jake Marisnick doubled to open the inning and he moved to third on Christian Yelich's groundout to second.
Miami's chance to pull even was denied when Stanton struck out and Jones tapped to second.
Jordan held the Marlins scoreless until the sixth inning when Stanton laced an RBI double to right-center. Yelich, who singled, raced home to trim Washington's lead to 2-1.
Craig Stammen relieved Jordan with two outs in the sixth, and retired Jarrod Saltalamacchia on a grounder to first, stranding the potential tying run at second.
In the seventh, the Nationals added an insurance run on Span's fielder's-choice RBI off Mike Dunn.
Up next: The Nationals play their last home game this spring at Space Coast Stadium against the Mets on Tuesday at 1:05 p.m. ET on MLB.TV. Right-hander Stephen Strasburg will start for Washington in his final tuneup for Opening Day against the Mets.