The Raptors were tied against the Philadelphia 76ers with mere seconds left in regular time of game seven.
Jimmy Butler had levelled the scoreline in the dying moments with a layup, seemingly sending the game to overtime as the 76ers then fouled Leonard.
With one possession left, and the score tied at 90-90, Leonard received the ball on the edge of the three-point area before dribbling round to the corner, and unleashing an off-balance jump shot.
The ball bounced four times on the rim before sinking through the basket after the buzzer, sending the Raptors through to the Eastern Conference Finals.
The crowd in Toronto was sent into bedlam, as fans poured onto the court to celebrate with Leonard and the rest of the team.
The Raptors will now go through to the Eastern Conference finals against the Milwaukee Bucks, who dismantled the Boston Celtics in five games in the semi finals.
Leonard has been a standout performer for the Raptors in the playoffs in his first year in Toronto.
The forward is averaging 31 points-per-game, with 3.6 assists and 6.6 defensive rebounds.
Last year, he only played nine games after suffering injuries, and he admitted after the game that he struggled to cope in his final year with the San Antonio Spurs.
“That felt great, we work on that every day, driving to the baseline.
“I missed the last one, it was short, so I just wanted to put it up in the air and it got the shooters bounce.
“Man, it’s a lot of hard work, I just shot it as hard as I could.
“I just tried to play hard in every possession, I just wanted to leave it all on the court in case we came up short tonight.
“Last year was a very down year for me. I was going through a lot with everything that was going on.
“God is good, I prayed every day and I’m healthy and able to play basketball.
“You can just see what he does for you.
“The Bucks are a very great team, they led the league this year, the only team with 60 wins, they’re a tough matchup. Hopefully this game seven prepared us for them.”