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 essentially became a five-lap shootout as the safety car was deployed after Maximilian Gunther’s Maserati MSG came to a stop heading into Massenet.
The German suffered damage on the run-up Beau Rivage, having tried to overtake Dan Ticktum’s NIO 333 machine which had a damaged front wing.
With energy saved behind the safety car and no added laps, the remaining tours were flat-out with Evans unable to find a way back past.
He was denied the chance of a last-lap overtake 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 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.
Sacha 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.
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.
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 the Porsche driver 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
| Cla | # | Driver | Team | Gap |
|---|---|---|---|---|
| 1 | 37 |  Nick Cassidy |
 Envision Racing |
|
| 2 | 9 | Mitch Evans |
Jaguar Racing |
0.390 |
| 3 | 27 | Jake Dennis |
 Andretti Autosport |
1.017 |
| 4 | 23 |  Sacha Fenestraz |
 Nissan e.dams |
2.148 |
| 5 | 5 | Jake Hughes |
McLaren |
2.788 |
| 6 | 33 | Dan Ticktum |
NIO Formula E Team |
3.368 |
| 7 | 25 | Jean-Eric Vergne |
DS Penske |
4.374 |
| 8 | 16 |  Sébastien Buemi |
Envision Racing |
4.783 |
| 9 | 1 |  Stoffel Vandoorne |
DS Penske |
5.394 |
| 10 | 10 | Sam Bird |
Jaguar Racing |
6.469 |
| 11 | 94 |  Pascal Wehrlein |
Porsche Team |
6.705 |
| 12 | 48 | Edoardo Mortara |
 Maserati Racing |
7.624 |
| 13 | 11 |  Lucas di Grassi |
 Mahindra Racing |
8.576 |
| 14 | 4 |  Robin Frijns |
Team Abt |
9.620 |
| 15 | 3 | Sergio Sette Camara |
NIO Formula E Team |
10.684 |
| 16 | 13 |  Antonio Felix da Costa |
Porsche Team |
11.141 |
| 17 | 58 | René Rast |
McLaren |
12.295 |
| 18 | 17 | Norman Nato |
Nissan e.dams |
13.423 |
| 51 | Nico Müller |
Team Abt |
2 Laps | |
| 7 | Maximilian Gunther |
Maserati Racing |
8 Laps | |
| 8 | Oliver Rowland |
Mahindra Racing |
11 Laps | |
| 36 | Andre Lotterer |
Andretti Autosport |
28 Laps | |
| View full results | ||||
