"It's an amazing day for the team, myself and really happy to have won here again," said the winner, who told reporters he did not accept victory was in the bag until the very last corner of the last lap.
"It's Monaco. You never get ahead of yourself here because you get bitten in the arse," he added.
The top five were split by just 4.1 seconds, running nose-to-tail through the hairpin, with Webber's team mate and double world champion Sebastian Vettel fourth ahead of McLaren's Lewis Hamilton.
Alonso has 76 points with Vettel and Webber both on 73.
In Image: Red Bull driver Mark Webber of Australia, leads Mercedes Grand Prix driver Nico Rosberg of Germany, during the Monaco F1 Grand Prix