The reigning champion's winning margin stood at 27.9s by the end of the 78-lap race, but that only told part of the story after rain threatened to ruin his day on lap 52. Although the rain was bad news for Verstappen it turned a traditional Monaco procession into a wild ride for the final third of the race as the drivers struggled for grip around the soaked 3.3 kilometre circuit.
Alonso and Aston Martin, who were running second, had a shot at victory had they committed to intermediate tyres on lap 54 before Verstappen had the chance to pit, but by opting initially for slick tyres they scuppered their chance as Alonso had to pit again the following lap to correct the mistake and fit intermediates.
That meant Verstappen switched to intermediates on lap 55 before Alonso and returned to the track in the lead, building with the Red Bull driver building his advantage over the remaining laps as the rain eased off. It's not entirely clear if Alonso would have secured the lead with a perfectly timed stop for intermediates, but with Verstappen using the barriers to slow himself for Portier on lap 55 as he got caught on slick tyres in the rain, it would have ratcheted up the pressure on Red Bull.
The conditions proved tricky for the entire field, with several mistakes in the middle part of the race, but Alonso's teammate Lance Stroll was the only driver to retire after damaging his front wing at the tight Fairmont Hairpin.
Verstappen's fourth victory of the season saw him extend his lead to 39 points in the championship standings after teammate Sergio Perez started last following his crash in qualifying and struggled throughout the race to finish 16th.
Esteban Ocon took a memorable podium for Alpine after holding off the Mercedes of Lewis Hamilton in the closing stages. Ocon started third after an impressive qualifying performance on Saturday and absorbed pressure initially from Carlos Sainz in the dry and then Hamilton once the Ferrari driver made a mistake when the rain started to fall and was shuffled backwards in the pit stops.
George Russell secured fifth behind Hamilton in fourth despite a five-second penalty for rejoining from a mistake at Mirabeau and causing a collision with Perez.