Nate Thompson and Ryan Dzingel also scored for the Senators (4-1-3). Craig Anderson made his seventh start of the season and had 28 saves for the win.
James van Riemsdyk, Auston Matthews and William Nylander replied for the Leafs (6-2-0) while Frederik Andersen made 29 saves.
It took five tries but the Senators were able to collect their first win on home ice this season, although they did collect a point in three of the previous four losses. Included in that was a 5-4 overtime loss to the New Jersey Devils when they had a 4-2 lead after 40 minutes on Thursday.
The loss also became the first blemish on the road record of the Leafs as they had entered the game with three road wins in three attempts.
Trailing 3-0 entering the final period, the Leafs struck quickly with goals from van Riemsdyk and Matthews at six minutes and 8:34 respectively, but Stone gave the Senators a two-goal cushion just seven seconds after Matthews’ goal for a 4-2 Senators lead.
Nylander scored on the power play at 12:52, pulling the Leafs to within a goal but they would get no closer. Brassard scored at 16:30 and Stone added an empty netter at 18:21.
On the strength of a first-period goal and two more in the second the Senators were able to take that apparent comfortable 3-0 lead into the third.
Thompson collected his first goal as a Senator when he smacked in a rebound at 12:01 of the first period as Andersen had his back to the puck and his head in the net following a mass scramble in front of the goal.
That advantage was increased to 2-0 when Dzingel tipped an Erik Karlsson point shot off the glove of Andersen and in at 14:20 of the second period.
The Senators kept pressing and made it a 3-0 game with just 12 seconds to play in the second period when Brassard took a cross-ice pass from Bobby Ryan off his skates and then beat Andersen on the stick side.
Notes; Mark Borowiecki was a scratch for the Senators Saturday while Eric Fehr, Josh Lewo and Calle Rosen were scratches for the Leafsâ€¦Craig Anderson became the first goaltender to appear in 300 games with the Ottawa Senatorsâ€¦The Senators host the Los Angeles Kings Tuesday in the fourth game of a five-game home stand. The Leafs will host those same Kings a night earlier at the ACC.