Wellemeyer (3-4) scattered four hits and gave up two runs to pick up the win and his two-out single started the Giants' four-run outburst in the fifth. Wellemeyer scored on Edgar Renteria's single, and then Renteria and Andres Torres scored on a double by Freddy Sanchez -- who then came in on a double by Pablo Sandoval.
Washington scored two in the seventh, which opened with three consecutive singles off Wellemeyer. Josh Willingham drove in Ryan Zimmerman with a base hit that sent Wellemeyer to the showers and Ian Desmond drove in the Nationals' second run with two outs off Sergio Romo.
Brian Wilson struck out the side in the ninth for his 11th save.
Livan Hernandez (4-3) took the loss, giving up four runs on seven hits over five innings.