Martin Frk broke a tie with 4:41 remaining in the third period and Gabriel Vilardi had a goal and an assist for the Kings (29-35-6), who are 10-2-1 in their past 13 games. Cal Petersen made 24 saves.
"It's selfish for me to say because I'm one of the new faces coming in, but we've won seven games," Vilardi said. "That means we're obviously doing something right. It feels pretty good to win hockey games. That's what we want to do."
WATCH: [All Senators vs. Kings highlights]
Bobby Ryan and Jayce Hawryluk scored, and Craig Anderson made 36 saves for the Senators (25-34-12), who were playing the second game of a back-to-back. They lost 5-2 at the Anaheim Ducks on Tuesday.
"I thought, overall, we played a pretty solid game," Anderson said. "I think we were doing a lot of right things, a lot of good things out there. The puck didn't bounce our way there, but we had a couple of good opportunities there at the end to tie it up and we just weren't able to capitalize."
Frk scored on a 3-on-2 after Drew Doughty broke up a shorthanded rush in the final seconds of a Kings power play.
Vilardi tied it 2-2 on the power play at 9:12 of the third period with a wrist shot through traffic. He has seven points (three goals, four assists) in his first 10 NHL games, including four points (two goals, two assists) during a three-game point streak.