Toews took a pass from Patrick Kane just inside the Ottawa zone, skated the puck to center ice before re-entering the zone, went around the net, and scored with a wrist shot from the high slot past goalie Marcus Hogberg.
"We really can't afford to lose many games right now," said Toews, who had two assists. "We weren't too happy with our start. All the guys came in here (after the first period) -- it wasn't just a few of the guys, it was everyone -- and kind of acknowledged that we had a lot better than that."
WATCH: [All Blackhawks vs. Senators highlights]
Dominik Kubalik scored twice to extend his goal streak to five games for the Blackhawks (21-20-6), who came back from down 2-0 for their second straight win and third in five games. Robin Lehner made 20 saves.
"It was a pretty bad game out there, to be honest," Lehner said. "Not much happened. I don't think we played that great, they didn't play that great. They got a couple bounces, but you know what, we stuck with it."
Connor Brown had a goal and an assist for the Senators (16-22-8), who have not won in eight games (0-4-4), their longest streak of the season. Hogberg made 27 saves.
"I don't think we worked hard enough," Ottawa coach D.J. Smith said. "It's simple. I just think they outworked us for the last two periods.
"We started to play their game, in the second period, especially. They quick-upped us a ton, and their big guys had the puck way too much. If we put it behind them and grind them and make them play some defense, they don't get as much offense."