After the Nationals went ahead 4-2 in the top the ninth, the Dodgers forced an extra inning by answering with Juan Uribe's RBI double and a run off a wild pitch uncorked by Henry Rodriguez.
Jamey Wright (1-0) blanked Washington in the top of the 10th and Kemp greeted Tom Gorzelanny (1-1) in the next half-inning with his MLB-best 11th homer of the year over the right-field wall.
Kemp's walk-off clout gave the National League West-leading Dodgers their 15th win in 21 games to start the season.
Bryce Harper, the first overall draft pick in 2010 draft, marked his major league debut with a sacrifice fly and Adam LaRoche homered for the Nationals, who dropped their third straight.
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