PSG's troubles in front of goal this season have often cost them in the Champions League, and they were thwarted throughout this game by a combination of poor finishing and some fine saves by Monaco goalkeeper Philipp Koehn.
However, it was ultimately a deserved victory for Luis Enrique's team in Qatar, as PSG won the trophy -- the French equivalent of a Super Cup or England's Community Shield -- for the 11th time in the last 12 seasons.
"It was a great game between two teams who play good football," said Luis Enrique.
"The result is well deserved and reflects our domination... against one of the best teams in France and in Europe."
This fixture was supposed to be staged in Beijing on August 8 as the curtain-raiser to the French campaign.
However, it ended up being called off by the Chinese authorities, and Qatar -- which owns PSG via Qatar Sports Investments -- stepped in to stage the game.
It took place at Doha's Stadium 974, which hosted matches at the 2022 World Cup held in Qatar and which was not full for this match.
PSG won a clean sweep of the domestic honours in France last season and were looking to claim the first leg of another treble in this campaign.
A 4-2 win away in Monaco last month, when Dembele scored a double, allowed them to finish 2024 with a 10-point lead at the top of Ligue 1 from both Marseille and the principality club.
Key month
When Desire Doue pounced on a slack pass out by Koehn and sent a shot off the bar on eight minutes, it looked like it was going to be a long evening for Monaco.
Vitinha sent a shot just wide and Lee Kang-in had a thunderous strike tipped over by Koehn, although Monaco also threatened when a Maghnes Akliouche hit was turned away by Gianluigi Donnarumma.
Eliesse Ben Seghir also tested PSG's Italian goalkeeper shortly after the break, and Vanderson hit the post from a tight angle for Monaco, but PSG stepped up the pressure as the second half went on.
The game was nevertheless heading for penalties before the goal came in the 92nd minute, as substitute Fabian Ruiz was released down the left and sent a low ball across the penalty area for Dembele to convert at the far post.
It is the perfect start to a huge month for PSG, which will notably see them face Manchester City in a crunch Champions League fixture on January 22.
Monaco also have a key few weeks ahead of them, including games against Aston Villa and Inter Milan as they too seek to progress to the knockout stage of the Champions League.
"A goal in the last minute always feels like being stabbed in the back," said Monaco's Austrian coach, Adi Huetter.
"But being honest, PSG deserved to win. They had more chances."
