Amid a sudden burst of heavy rain that scurried south from the Manhattan skyline, the circuit around the Brooklyn Cruise Terminal quickly became saturated with water - creating treacherous conditions and building a puddle at Turn 6.
Cassidy then splashed through the puddle and aquaplaned, spilling his Envision car into the barriers at a vast rate of knots - with Lucas di Grassi and Stoffel Vandoorne following him into the barrier.
But owing to the considerable level of clean-up required by the track marshals, race control called an end to the race - picking up the classification from the end of lap 29 to declare Cassidy the victor.
The New Zealander thus successfully converted the win from pole, where he set his car up on the starting grid pointing to the inside line in a bid to cover off Vandoorne.
But that proved unnecessary as Cassidy grasped the best start, as Vandoorne floundered and lost places to di Grassi and Sebastien Buemi off the bat.
Although di Grassi was attempting to reel Cassidy in, he struggled to break into the half-second buffer that the Kiwi was keeping in reserve.
Cassidy also oversaw any assaults from di Grassi through the attack mode stage, keeping the Venturi driver at bay and also fending off a lap 24 tilt on fanboost.
Ironically enough, di Grassi's failure to make the most of his fanboost allowed Vandoorne to make the pass on the Brazilian with his own five-second boost of power.
But di Grassi rallied and put a move back on Vandoorne at the start of lap 28, which the rapidly catching Frijns then pounced on and stole the inside line to collect third place, as rain began to fall.
Cassidy had over a second on di Grassi amid the drizzle, with Frijns keeping tabs on the pair and looking to pick up any pieces - which ultimately came to pass on lap 30 when the rain intensified immediately.
The downpour then looked to have claimed Cassidy's victory chances as the Envision driver aquaplaned and speared into the wall, followed in by di Grassi and Vandoorne.
But Frijns cannily took his foot off the accelerator and stopped in time, leaving the corner with the lead of the race.
Edoardo Mortara also aquaplaned and knocked off his right-front wheel fender, but was able to come to a halt just before reaching the wreckage - pulling a 180-degree turn to rejoin behind Frijns.
In the meantime, Buemi was also a casualty of the rain while Pascal Wehrlein came to a halt on the exit of the corner and was subsequently collected by Sam Bird and Jake Dennis - which let Mitch Evans trickle through.
The race was thus red-flagged to clean up the mess, as the deluge immediately halted as the remaining cars tiptoed back to the pits.
But given the damage to the wall, the race was called off - which restored Cassidy to the top step.
Di Grassi's later pass on Vandoorne and successful preservation of second from Frijns ensured that the Brazilian secured his best finish of the 2021-22 season, as Frijns put two Envisions on the podium.
Vandoorne was left to be content with fourth, while Mortara rescued a strong fifth place to retain his title lead and lessen the impact from the Belgian in the points.
Like his last two races in Jakarta and Marrakech, Mortara held off on taking his second and final attack mode and used it to ignite a charge back through the order after dropping to 10th, manoeuvring his way through the lower half of the points.
Buemi, despite also getting caught out in the rain, bagged sixth place ahead of Wehrlein - whose incident with Bird and Dennis will be investigated.
Dennis collected eighth place ahead of Bird, while Nyck de Vries completed the top 10 - but had been fourth at the red flag before the results were taken from two laps prior.
Evans got 11th after qualifying 14th, but was involved in a first-lap Turn 6 pile-up involving Jean-Eric Vergne - which put the Frenchman well behind the rest of the field and gave both drivers a significant dent to their title hopes.
Maximilian Guenther was 12th, and also faces an investigation along with Andre Lotterer as they were involved in an incident with Antonio Felix da Costa - which proved terminal for the Portuguese driver.
Dragon endured a miserable race, with Sergio Sette Camara unable to take to the start as a legacy of his impact damage from qualifying, while Giovinazzi was caught up in the opening lap skirmish between Vergne and Evans and had to pit and retire.
New York City E-Prix I - Race Results
| Cla | Driver | Team | Laps | Time | Gap | Interval |
|---|---|---|---|---|---|---|
| 1 |  Nick Cassidy |  Virgin Racing | 29 | 35'04.095 | ||
| 2 |  Lucas di Grassi |  Venturi | 29 | 35'05.218 | 1.123 | 1.123 |
| 3 |  Robin Frijns | Virgin Racing | 29 | 35'05.766 | 1.671 | 0.548 |
| 4 |  Stoffel Vandoorne |  Mercedes | 29 | 35'07.788 | 3.693 | 2.022 |
| 5 |  Edoardo Mortara | Venturi | 29 | 35'08.829 | 4.734 | 1.041 |
| 6 | Sébastien Buemi |  DAMS | 29 | 35'09.665 | 5.570 | 0.836 |
| 7 | Pascal Wehrlein | Porsche Team | 29 | 35'09.878 | 5.783 | 0.213 |
| 8 | Jake Dennis |  Andretti Autosport | 29 | 35'10.690 | 6.595 | 0.812 |
| 9 | Sam Bird | Jaguar Racing | 29 | 35'11.302 | 7.207 | 0.612 |
| 10 | Nyck de Vries | Mercedes | 29 | 35'11.598 | 7.503 | 0.296 |
| 11 | Mitch Evans | Jaguar Racing | 29 | 35'12.700 | 8.605 | 1.102 |
| 12 | Maximilian Gunther | DAMS | 29 | 35'17.858 | 13.763 | 5.158 |
| 13 | Andre Lotterer | Porsche Team | 29 | 35'18.479 | 14.384 | 0.621 |
| 14 | Oliver Rowland |  Mahindra Racing | 29 | 35'19.019 | 14.924 | 0.540 |
| 15 | Alexander Sims | Mahindra Racing | 29 | 35'20.743 | 16.648 | 1.724 |
| 16 | Oliver Turvey | NIO Formula E Team | 29 | 35'23.802 | 19.707 | 3.059 |
| 17 | Dan Ticktum | NIO Formula E Team | 29 | 35'29.407 | 25.312 | 5.605 |
| 18 | Jean-Eric Vergne | Techeetah | 29 | 35'57.113 | 53.018 | 27.706 |
| 19 | Oliver Askew | Andretti Autosport | 29 | 36'05.377 | 1'01.282 | 8.264 |
 Antonio Felix da Costa | Techeetah | 28 | 34'02.249 | 1 Lap | 1 Lap | |
 Antonio Giovinazzi | Dragon Racing | 0 | ||||
| Sergio Sette Camara | Dragon Racing | 0 | ||||
| View full results | ||||||
