The night started off on the wrong foot for the Jays, who lost starter Brandon Morrow after just nine pitches in the first inning with a strained left oblique. He's listed as day-to-day.
Rookie sensation Bryce Harper had three singles and a walk and Rick Ankiel hit a solo homer for the Nationals, who won their fourth in a row against a Toronto bullpen that was forced to use five pitchers in Morrow's absence.
Colby Rasmus and Yan Gomes homered for the Blue Jays, who were held to three hits and two runs by Jackson (3-3) in the opener of a three-game interleague series.
Jackson departed after Edwin Encarnacion led off the ninth with a double. Sean Burnett replaced Jackson and promptly gave up a pinch-hit home run before getting J.P. Arencibia to ground out to end the game.
Morrow left in discomfort while facing a 2-1 count against Harper in the first. He walked from the field to the dugout and down the stairs to the clubhouse unassisted after being checked out by trainers and manager John Farrell.
Chad Beck came on for Morrow and gave up Harper's single to score Steve Lombardozzi, who was at second after a leadoff double.
Harper went to third when first baseman David Cooper was charged with an error on Ryan Zimmerman's hard-hit ball. Michael Morse followed up with a sacrifice fly to score Harper.
Beck, who has been closing with triple-A Las Vegas and has usually been limited to one inning with the Jays, lasted 3 2-3 innings before being replaced by Luis Perez with two out in the fourth. Beck allowed three runs, two earned.
The Blue Jays (31-30) got one back in the bottom of the first on the ninth homer of the season by Rasmus.
LaRoche hit his 11th home run of the campaign to make it 4-1 with two outs in the third after Harper walked.
The Nationals (36-23) scored once in the seventh after loading the bases on three singles, starting with a bunt down the third-base line by Lombardozzi.
Ian Desmond scored Lombardozzi with a two-out single against Jason Frasor, but the Nationals left the bases loaded.
Ankiel, who entered the game in the seventh to play centre field, hit his fourth homer of the season to lead off the eighth against Francisco Cordero.
Notes: Attendance at Rogers Centre was 18,513. ... Washington centre-fielder Roger Bernadina came out of the game in fourth inning with a strained right hamstring after going first to third on a wild pitch. He was replaced by Xavier Nady. Harper moved from right field to centre and Nady played right field. Harper moved back to right when Ankiel entered and replaced Nady. ...Harper was thrown out at third by right-fielder Jose Bautista in the fifth while trying to go first to third on a single ... Vladimir Guerrero, who is on a minor-league contract at triple-A Las Vegas, was back in the starting lineup Monday after missing one game after he was hit by a pitch on the left hand on Saturday. He was batting .214 (6-for-28) with two RBIs in seven games since being called up from single-A Dunedin on June 2 ... Washington's Chien-Ming Wang (1-2, 5.11 earned-run average) will face Toronto's Henderson Alvarez (3-5, 3.76 ERA) on Tuesday.