Fans at Maple Leaf Square watched as Boston scored the first goal of Game 7 and also as the Bruins then fell behind the Leafs by a score of 4-1.
But the Bruins scored three goals in the third period and forced the game into overtime.
One more goal from Boston sealed the deal and the Leafs' season ended in a 5-4 loss.
The Bruins will head to the second round of NHL playoffs, while the Leafs head home for the season.
Fans in Toronto were stunned by the loss that seemed unfathomable earlier in the evening when the Leafs had run up a three-goal lead over the Bruins.
"I'm proud of my Leafs, they played good. But the game today, 4-1 we were up, we shouldn’t have let that lead go," one fan told the CBC’s Natalie Kalata following the loss.
The scrappy Leafs came back from a 3-1 series deficit to claim back-to-back wins against Boston and tie the series. Then came the unfortunate events of Monday night.
Some Toronto fans cried in the aftermath of the loss in overtime, others looked to what the team might achieve in future and what it had done during its brief first-round playoff run.
On Twitter, there was no denying the heartbreak that some fans were feeling.
The police were out on downtown Toronto streets to ensure there weren't any problems. Mounted police and officers on foot and on bicycles helped disperse remaining crowds.
Officers dealt with a few rowdy fans who threw beer bottles after the loss. Some pulled the tails and slapped the sides of the horses that officers were riding on.
Some Toronto officers, like their fellow Leafs fans, tweeted that the playoff run had been good while it lasted.
The Leafs last won the Stanley Cup in 1967. With Monday night's loss, that championship drought continues.
Before Toronto’s entry to the first-round of this year's playoffs, the team had not even been to the post-season in nearly a decade.