Drake Batherson had an assist on DeBrincat's first goal to extend his point streak to 12 games for the Senators (16-16-3), who have won their past two games. Giroux had two assists, and Cam Talbot made 20 saves in his 400th NHL start.
DeBrincat, acquired in a trade with the Chicago Blackhawks on July 7, has four goals in his past three games and 13 for the season.
"In any new system, it takes some time," Pinto said. "He's in a new spot and you knew he was going to start scoring. It was just a matter of time. He's such an elite player and he's just getting more and more comfortable and he's just showing everyone what he can do. I'm not surprised at all."
Alex Ovechkin scored his 23rd goal of the season for the Capitals (20-13-5), who had won five straight. Darcy Kuemper made 41 saves.
"I think we got away a little bit from why we've been having such a good month here," Kuemper said. "I think it will be easy to recognize and turn things around. Not worried from that standpoint. Got to make sure we are better next game."
Washington was outshot 16-4 in a scoreless first period.
"Kuemper gave us a chance, especially in the first period," Capitals coach Peter Laviolette said. "I thought he made some big saves and then he had to be on point all night for different reasons."
Ovechkin gave the Capitals a 1-0 lead at 1:15 of the second period when he received a stretch pass from Erik Gustafsson at the blue line, skated in alone and scored on a wrist shot through the five-hole.