Lincecum was not charged with a loss in his previous appearance despite giving up five runs in five innings against Arizona. He suffered his first setback of the season against the Nationals, however, struggling over 4 2/3 frames.
Lincecum (5-1) gave up six runs on six hits and walked five in his shortest outing of the year.
Desmond's RBI single finished off a three-run third and he contributed a two-run double during a three-run sixth.
The Nationals, even though they are tied for last place in the National League East, moved within three games of first in the competitive division and advanced to a game above .500.
Washington built a 6-0 lead and then held on to give starter Luis Atilano the victory. Atilano (4-1) surrendered two runs on four hits in 5 1/3.