Filip Gustavsson made 22 saves for the Senators before Mads Sogaard played the third period and stopped six shots.
Michael Bunting had a hat-trick for Toronto while David Kampf also scored as Peter Mrazek turned away 19 shots for the shutout.
"We weren't very good," Senators head coach D.J. Smith said. "I thought Parker Kelly gave us a lot of energy and played his game for the most part but outside of that, thank goodness it's an exhibition game.
"We didn't have the puck a lot, we didn't shoot a lot. We just didn't play anything like the way we want to play."
Toronto had the lone goal in the first and it came through Kampf at 13:25 who was on hand to fire home a low shot through Gustavsson after Carl Dahlstrom's point shot had hit a skate out front.
The lead was 2-0 at 15:41 of the second as with Chris Tierney in the box for cross-checking, Bunting buried Josh Ho-Sang's centering pass past Gustavsson for his first of the night. He had his second power play marker 2:11 later as he got a slight piece of Rasmus Sandin's point shot to give the Maple Leafs a 3-0 advantage.
"That was a tough one tonight," Nicholas Paul said. "We come into the dressing room after two periods and have 12 shots. It's not good. You can say it's a preseason game and get the rust off but there's a lot of things we learned off that game and obviously you want to flush it, but there's some things that we've got to get better at."
Bunting completed his hat-trick with his third power play goal of the night with 1:17 to play.
"You don't want that to happen in the regular season where you play like that," Smith said. "That's just not the way we play. We didn't play at all the way we want to play. We had almost zero o-zone time and when you don't have that, you don't break out well and everything else goes awry from it, so we've got to shoot a lot more pucks and get on goalies but like I said, I'm glad it was an exhibition game."
Clark Bishop left the game in the second period with a lower-body injury and did not return.
Post-game, the teams held a shootout that was won by Toronto 2-1. Connor Brown scored for the Senators while Tim Stützle and Colin White were unsuccessful. Toronto's Nikita Gusev and Nick Robertson scored.
The Senators are back in action Friday when they host Montreal in preseason action at 7 p.m.
