The Envision Racing driver took a lights-to-flag victory after an imperious performance in treacherous conditions having rebuffed a strong challenge from Jaguar's Evans.
The win and sixth-place finish for Sebastien Buemi means Envision has provisionally claimed the teams' championship having been in a battle against the Jaguar entering the final race of the season.
Cassidy and Evans dominated proceedings, with newly crowned champion Jake Dennis completing the podium more than 16s behind Cassidy.
The last race of the Formula E season was finely poised on Sunday at the ExCeL London's unique indoor/outdoor circuit, with Cassidy just taking pole from Saturday's London E-Prix winner Evans.
As well as pole, Cassidy more importantly secured three points which pivotally, moved Envision clear of Jaguar in the teams' standings prior to the race.
The field began the scheduled 34-lap race behind the safety car ue to heavy rain outside the main arena, with race director Scot Elkins stating that the field would circulate for several laps before lining up on the grid for a standing start.
After three laps the safety car headed to the pits with drivers set to take up their positions on the grid, but moments later the safety car was redeployed before the cars came to a stop.
Two more laps were run under caution, but with conditions still not improving the decision was taken to red-flag the race.
Following a 30-minute delay in which sweeping machines and marshals took to the track to try and disperse water, the field filed out of the pits behind the safety car.
Race director Elkins stated that the field would perform two laps again to reassess the conditions, but with heavy standing water still remaining a second red flag was shown with cars entering the pitlane once more.
After a second lengthy delay and with the rain easing, the race resumed at 1830 local time with green action running getting underway for the first time on lap eight with a rolling start.
Cassidy and Evans immediately managed to pull clear of the chasing pack, with Dennis stuck behind Norman Nato for two laps before moving ahead when the Nissan driver took his first Attack Mode.
The top three, all of whom were title protagonists in yesterday's opening race, took both their Attack Mode activations in the first eight laps as energy saving became somewhat irrelevant in the wet conditions.
The biggest gap between the leading pair only briefly exceeded 3s, with the gap generally stabilising around the 2.5s mark as Dennis fell more than 10s behind after just 10 laps of green flag running.
The extended safety car period meant four extra laps were added to the total race distance, during which time Evans began to drop back from Cassidy, eventually finishing nearly 5s behind at the finish.
Dennis finished a further 11s behind but well clear of Nato, with DS Penske's Stoffel Vandoorne claiming fifth after a solid performance.
Behind, the second Envision of Buemi had moved ahead of Jaguar's Sam Bird around the outside into Turn 1 on lap 26 for sixth, solidifying Envision's spot at the top of the teams' championship by 12 points.
Abt Cupra's Nico Muller finished eighth having come under intense pressure from NIO 333's Dan Ticktum, as Porsche's Pascal Wehrlein claimed 10th, with all 22 cars incredibly finishing after no yellow flag periods.
| Pos | Driver | Team | Gap |
|---|---|---|---|
| 1 |
 Nick Cassidy |
Envision Virgin Racing | |
| 2 |
Mitch Evans |
Jaguar Racing | 4.934 |
| 3 |
 Jake Dennis |
Avalanche Andretti Formula E | 16.295 |
| 4 |
 Norman Nato |
Nissan Formula E Team | 24.819 |
| 5 |
 Stoffel Vandoorne |
DS Penske | 26.290 |
| 6 |
 Sebastien Buemi |
Envision Virgin Racing | 27.406 |
| 7 |
Sam Bird |
Jaguar Racing | 29.376 |
| 8 |
Nico Müller |
ABT CUPRA Formula E Team | 30.304 |
| 9 |
Dan Ticktum |
NIO 333 Racing | 30.832 |
| 10 |
 Pascal Wehrlein |
TAG Heuer Porsche Formula E Team | 35.558 |
| 11 |
Edoardo Mortara |
Maserati MSG Racing | 36.615 |
| 12 |
René Rast |
NEOM McLaren Formula E Team | 38.160 |
| 13 |
 Sérgio Sette Câmara |
NIO 333 Racing | 40.295 |
| 14 |
Maximilian Günther |
Maserati MSG Racing | 51.140 |
| 15 |
Sacha Fenestraz |
Nissan Formula E Team | 51.918 |
| 16 |
 Antonio Felix da Costa |
TAG Heuer Porsche Formula E Team | 53.336 |
| 17 |
 Robin Frijns |
ABT CUPRA Formula E Team | 56.608 |
| 18 |
Lucas di Grassi |
Mahindra Racing | 58.064 |
| 19 |
Jake Hughes |
NEOM McLaren Formula E Team | 59.956 |
| 20 |
 Roberto Merhi |
Mahindra Racing | 1'02.506 |
| 21 |
Andre Lotterer |
Avalanche Andretti Formula E | 1'02.890 |
| 22 |
Jean-Eric Vergne |
DS Penske | 1 Lap |
