Nady connected for a two-run shot and Roberts added a solo blast, and Chris Young finished the four-run outburst with a sacrifice fly for the Diamondbacks.
Kennedy (4-1) limited the Dodgers to one run on four hits over a six-inning start, issuing only one walk and striking out eight as Arizona took the rubber match of a three-game series at Dodger Stadium.
J.J Putz earned his ninth save despite allowing a pair of base runners in the ninth.
The Dodgers' Ted Lilly (3-4) was touched for four runs on five hits over six innings.
Aaron Miles went 3-for-4 with an RBI for Los Angeles.