Cassidy led home Mitch Evans (Jaguar TCS Racing), having fended off his countryman until a late-race Safety Car made the win certain for the Envision Racing man, who was under severe duress from the factory Jaguar driver.
The 150mph game of chess ebbed and flowed as leaders vied for control and to set the pace but Cassidy's decisive early-race moves yielded the ultimate result, with the Kiwi placing his I-TYPE 6 perfectly around the outside of three at the Fairmont Hairpin on Lap 4 and managing to hit the front as early as Lap 7 through the first round of ATTACK MODE activations. Once his engineer gave the green light for a six-lap sprint finish, Cassidy didn't look back - despite the close attentions of Evans' factory Jaguar.
Evans had himself clambered from sixth on the grid to second at the chequered flag and was within touching distance of the Envision right to that Safety Car three laps from the race finish. That New Zealand one-two made it four wins on the spin - a new Formula E record for a single nation.
Jake Dennis (Avalanche Andretti) couldn't quite live with the lead pair but to his credit, he had torn through from 11th on the grid to make the final step on the podium - some feat.
Sacha Fenestraz, who thought he'd sealed Julius Baer Pole Position only for a post-session penalty to hand that honour to Jake Hughes (NEOM McLaren), steered his Nissan home to fourth, unable to compete with the lead trio's benchmark combination of speed and efficiency. Hughes followed him across the line, with Dan Ticktum hanging on despite a couple of late-race scrapes and some damage to his NIO 333 for sixth position.
Long-time Drivers' World Championship leader Pascal Wehrlein could only improve to 11th from 12th at the outset - the Porsche 99X Electric still has the performance within but unlocking it consistently over a lap and in qualifying appears to be an ongoing and potentially costly issue, especially with new standings leader Cassidy sealing two wins and four further podiums in the last six rounds.
Fellow title contender Jean-Eric Vergne recovered to seventh from the very back of the grid after DS PENSKE's tyre pressure infringements saw them disqualified from qualifying. Reigning champion and teammate Stoffel Vandoorne was also able to climb to the points with ninth in his DS.
All that left Cassidy 21 points clear of Wehrlein on 121 in the Drivers' table with Dennis third and Evans now fourth - Vergne doing enough to leave Monaco fifth in the running. Envision Racing now leap to the top of the Teams' standings, 14 points ahead of TAG Heuer Porsche, while Jaguar TCS Racing sits third.
As it happened...
Hughes led cleanly away from Fenestraz, Nato, Ticktum, Guenther and Mitch Evans the top six. Nick Cassidy was the big mover away from the line - the Envision Racing man jumping two spots to seventh.
On Lap 2, Rowland followed suit with a big lunge up the inside of Fairmont Hairpin - the Brit in the Mahindra having sliced his way through from 13th to sixth by the end of the tour and took the opportunity to leap for the first of his two mandatory 50kW ATTACK MODE boosts on Lap 3 having gained that early ground.
Come Lap 4, Cassidy had been shuffled back to ninth after his opening manoeuvres, but went side by side with Dennis and Wehrlein, going the long way around both on the outside of the Hairpin. His progress didn't stop there, with the Kiwi on an absolute mission. On Lap 5 he slipped by Evans and a tour later he saw off polesitter Hughes for fourth around the outside again, this time at Antony Noghès. A strong run over the line saw him breeze by Nato, too, for a net second spot and seven spots gained as it stood.
Lap 7 saw leader Fenestraz dive for ATTACK MODE - handing the lead to Cassidy, while Hughes' slip back through the top 10 continued - Dennis taking him at Turn 1 to leave the NEOM McLaren driver seventh. Cassidy and Nato followed through the ATTACK MODE loop next time around with Dennis profiting to pass title rival Cassidy for fouth on-track. Ticktum took his turn out front with the majority of the pack running level on usable energy remaining.
Another reshuffle through the first ATTACK MODE activations left Evans from Fenestraz and Cassidy out front on Lap 10, with Ticktum back down to fourth, Hughes, Nato, Guenther, Mortara and da Costa the top 10.
Evans briefly lost the lead with the second round of ATTACK MODE playing out, but did regain it from compatriot Cassidy in the Envision Racing run Jaguar I-TYPE 6 - the Jaguar powertrain again looking the part. The Jaguar driver made the leap through the loop for his second 50kW boost a lap later, handing the Envision driver the lead once again - the factory Jaguar driver also briefly sent back behind Dennis' Avalanche Andretti, though come the Nouvelle Chicane, he had forced his way back into second spot.
At the halfway stage, just 2.8 seconds split the top 10 runners - absolutely nothing between the lead pack with Cassidy, Evans, Dennis, Fenestraz, Guenther, Mortara, Ticktum, Hughes and Nato the drivers in the points as it stood.
Evans - a podium finisher here twice in the last two seasons - made the tail end of his final 50kW boost count by sweeping by leader Cassidy at Sainte Devote at the start of Lap 15, with a slight energy advantage on the sister I-TYPE 6, too.
A squabble at the harbourfront chicane on Lap 16 saw Bird, da Costa and Mortara come to blows in the fight for the last few points-paying positions - the Porsche driver once again suffering poor luck and coming off worst with the trio three wide and a nudge causing a puncture and all but ending the Portuguese' race. The lead then swapped again, with Cassidy mirroring Evans' move two laps prior at Turn 1. Three of the top four in the championship were fighting to decide the podium order, with neither of Cassidy, Evans and Dennis giving the other an inch.
On Lap 19, the game of chess at 150mph took another turn, with the pack biding their time and slowing to laptimes around the 1m37.000s bracket, having run as much as four seconds quicker just a handful of laps prior. Picking the moment to jump would, as in Berlin, once again be decisive. The entire top 10 sat within a percentage point of energy when Joseph Roca gave Cassidy the green light to go for it on Lap 20, with nine laps left to run.
As the race headed towards its climax, Ticktum and Nato got together at Rascasse, with the nose of Ticktum's NIO 333 machine catching on his front-right tyre. The Brit was able to hold sixth but Nato was forced into retirement.
With 21 laps in the books, Guenther's car came to a halt at Casino Square - Ticktum having moved to defend in front of the Maserati MSG Racing driver and the German running squarely into the back of the NIO 333.
The race went back green on Lap 23, with Evans and Dennis right with leader Cassidy when the Envision driver made the jump and the trio a second clear of fourth-placed Fenestraz. Cassidy put the hammer down and immediately set the then-TAG Heuer Fastest Lap.
The two New Zealanders then went flat out for the final five laps, with the Jaguar-powered pair able to leave Dennis in the Avalanche Andretti Porsche 99X Electric behind. The lead three set their personal bests in their fight for P1 but a shunt from a points-paying position for Nico Mueller (ABT CUPRA) at Turn 1 put paid to the race with things ending under the Safety Car.