The Kiwi had started ninth after missing out on the qualifying duels but stormed to the front of the pack just after the halfway point of the 29-lap race.
The race-winning move came into Sainte Devote on lap 18 as he overtook Mitch Evans, the Jaguar driver having jumped Cassidy at the same spot two laps earlier.
The race, which didn’t feature the extreme pack shuffling that has been a mainstay of other Formula E races this season, essentially became a five-lap shootout courtesy of a late safety car.
This was deployed after Maximilian Gunther’s Maserati MSG came to a stop heading into Massenet, the German having suffered damage on the climb up Beau Rivage when he tried to overtake Dan Ticktum for sixth.
The Briton’s NIO 333 machine was suffering from a damaged front-wing having made contact with the rear of Sacha Fenestraz’s Nissan the lap before into Rascasse, while his team-mate Norman Nato also retired with damage at the same time.
With energy saved behind the safety car and no added laps being applied, the remaining tours were flat-out with Evans immediately mounting pressure on Cassidy.
He was unable to produce a serious overtaking opportunity, though, and was denied the chance of a last-lap pass as a collision between Evans’ team-mate Sam Bird and Nico Muller left the Abt driver in the barrier at Sainte Devote.
The stranded car meant a second safety car was deployed as the race finished under caution, with Cassidy following on from his win in Berlin last month with his first success in Monte Carlo.
Jake Dennis completed the podium for Andretti Autosport having challenged Evans into the Nouvelle Chicane prior to the first safety car but was unable to find a way through.
Fenestraz took fourth after shadowing the leading trio for the second half of the race, the Nissan driver having been stripped of pole for exceeding the allowed energy output.
McLaren’s Jake Hughes, who had inherited pole and led early on, held onto fifth while Ticktum took sixth despite the damage to his front-wing having also led at one stage.
Jean-Eric Vergne impressively finished seventh after he and DS Penske team-mate Stoffel Vandoorne started on the back-row due to a qualifying infringement, with the reigning Formula E champion finishing ninth after both made steady progress.
Cassidy’s Envision team-mate Sebastien Buemi failed to feature near the front after starting 13th but finished eighth, as Bird took the final point despite his contact with Muller.
Pascal Wehrlein entered the race with a four-point lead over Cassidy in the standings, but struggled to break into the points at any stage in the race and eventually finished 11th.
His Porsche team-mate Antonio Felix da Costa also failed to score, finishing 16th after stopping to replace a punctured tyre.
Formula E Monaco E-Prix - race results (29 laps)
| Cla | Driver | Car | Gap/Retirement |
| 1 | Nick Cassidy | Envision | |
| 2 | Mitch Evans | Jaguar | +0.390s |
| 3 | Jake Dennis | Andretti | +1.017s |
| 4 | Sacha Fenestraz | Nissan | +2.148s |
| 5 | Jake Hughes | McLaren | +2.788s |
| 6 | Dan Ticktum | NIO | +3.368s |
| 7 | Jean-Éric Vergne | DS Penske | +4.374s |
| 8 | Sébastien Buemi | Envision | +4.783s |
| 9 | Stoffel Vandoorne | DS Penske | +5.394s |
| 10 | Sam Bird | Jaguar | +6.469s |
| 11 | Pascal Wehrlein | Porsche | +6.705s |
| 12 | Edoardo Mortara | Maserati | +7.624s |
| 13 | Lucas di Grassi | Mahindra | +8.576s |
| 14 | Robin Frijns | Abt | +9.620s |
| 15 | Sergio Sette Câmara | NIO | +10.684s |
| 16 | Antonio Felix da Costa | Porsche | +11.141s |
| 17 | René Rast | McLaren | +12.295s |
| 18 | Norman Nato | Nissan | +13.423s |
| Nico Müller | Abt | Accident | |
| Max Günther | Maserati | Accident | |
| Oliver Rowland | Mahindra | Accident damage | |
| André Lotterer | Andretti | Accident | |
