Ovechkin, who has seven goals in his past four games, is one goal behind Howe for second in League history, and 94 behind Wayne Gretzky for first.
"You know what? It was awesome," Capitals coach Peter Laviolette said. "It was just awesome to be on the bench and be a part of it, be a part of history. You work with somebody every day and they reach such an incredible milestone, it's an unbelievable accomplishment.
"Yeah, we said it after the game. That's an incredible accomplishment. When somebody has a career where they can get 200 goals, that's a heck of a career. But 800 is something and I think everybody is just really happy for him."
Mantha had a goal and an assist, and Charlie Lindgren made 26 saves for Washington (15-12-4), which has won five straight.
Petr Mrazek, who was playing his first game since Dec. 3 because of a groin injury, made 23 saves for Chicago (7-16-4), which has lost five in a row (outscored 16-4) and 12 of 13 (1-11-1).
"Obviously, pretty incredible milestone. One of kind," Blackhawks captain Jonathan Toews said. "In some sense, when we're out of this moment, it's one of those things you'll look back on. He's one of the great players of all time, so all due respect to him in scoring three goals to do it, getting himself an ovation and the 'Ovi' chant in our building.
"Aside from the milestone, it didn't feel great on our side, but lots of respect for him."
After Ovechkin gave the Capitals an early 2-0 lead, Tyler Johnson cut it to 2-1 at 19:00 of the first period from the low slot. Johnson was playing his first game since Oct. 25 because of an ankle injury.
"Obviously, it's always better when you're playing games," Johnson said. "I'm not a big fan of being out. It was nice to be back with the guys, but obviously not quite the result we wanted."