After delays for the wet weather to recede to allow a safe race to take place, Cassidy composed himself to stride to a comfortable race win - heading home fellow New Zealander Mitch Evans and new champion Jake Dennis (Avalanche Andretti) to deliver the Teams' crown the way of Jaguar customers Envision Racing, ahead of the factory Jaguar TCS Racing squad.
Cassidy led from pole, lights-to-flag and set the TAG Heuer Fastest Lap of the race, keeping Evans at arm's length throughout, while the lead pair drew out some 10 seconds over next-best Dennis with conditions as tough as it gets. Not only did the Envision Racing-run Jaguar I-TYPE 6 have the pace advantage but it also looked better on energy, too. That result left Cassidy as runner-up in the Drivers' table.
This allowed Sebastien Buemi in the sister Envision to steer to sixth, fending off Sam Bird in the other Jaguar TCS Racing car to make doubly sure of Teams' honours.
Norman Nato (Nissan) and outgoing champion Stoffel Vandoorne (DS PENKSE) were sandwiched between the front three and the squabbling Buemi and Bird, while Nico Mueller (ABT CUPRA) finished eighth.
Dan Ticktum, on home soil in the NIO 333, made a smart early move on Maserati MSG Racing's Edo Mortara to break into the top 10 and steered to an eventual ninth, ahead of TAG Heuer Porsche's Pascal Wehrlein.
That final round of Season 9 left Envision Racing the Teams' World Champions - their first title, with a prior best of third. They beat out the factory Jaguar TCS Racing team by 12 points. Porsche customers Avalanche Andretti also edged the factory TAG Heuer Porsche team to third, by 10 points.
Dennis wound up 30 points clear of Cassidy in the Drivers' running, with Evans third and Wehrlein fourth.
As it happened...
After a long delay and eight laps behind the Safety Car with wet weather getting the better of things, Cassidy led things away into the first corner with Evans and Nato in tow - the field fishtailing around the second sector once they'd left the indoor section of the track.
Stoffel Vandoorne and Sam Bird immediately jumped for their first of two mandatory 50kW ATTACK MODE boosts on Lap 9 - opting, with several other drivers outside the top 10, to ensure they'd get them in before any further stoppages came into play. Evans followed from P2, with Nato, Buemi, Mueller and Ticktum a lap later on Lap 10.
Cassidy headed Evans, Dennis, Nato, Vandoorne, Bird, Buemi, Mueller, Wehrlein and Ticktum the top 10 - the lead pair pulling out 2.5 seconds in the trickiest of conditions on third-placed driver Dennis. The Kiwi in P1 then jumped for his first ATTACK MODE activation on Lap 11, retaining his advantage at the head of the pack.
On home soil, NIO 333's Dan Ticktum made a move stick on Maserati MSG Racing's Edoardo Mortara on Lap 15 as the Swiss-French-Italian got sideways out of Turns 4 & 5, opening the door for 10th and points.
Cassidy was really putting the hammer down out front, mastering the ultra-slippery conditions. He'd pulled out three seconds on second-placed Evans come Lap 17, with the Jaguar TCS Racing driver himself some seven seconds clear of third-placed man Dennis.
On Lap 19, the best racing was to be had between three champions between Vergne in 16th, di Grassi 17th and da Costa in 18th as the former got a nudge from di Grassi with the trio inseperable as they looked to climb the order - just 0.8 seconds between all three of them. That battle came to a swift end with an issue for the DS PENSKE man allowing di Grassi and da Costa to sail by on Lap 23 as Vergne fell to the back of the pack - sustaining damage or a puncture in that earlier contact that needed recfiying via a pitstop.
Cassidy cleared up the use of his second ATTACK MODE, keeping hold of a three-second lead out front while setting a string of TAG Heuer Fastest Laps. He'd also got a percentage point or two in-hand on Evans and Dennis behind. It looked comfortable for the Envision Racing driver, with the team in search of Teams' honours. The Kiwis had some 11 seconds on Lap 26 on the guys behind - they were in a class of their own.
Bird put the hammer down in the battle with Buemi for sixth spot as Jaguar TCS Racing sensed the need to push - behind Envision in the Teams' running as it stood with nine laps to go. The Swiss denied Bird the chance to get past - later on the brakes into Turn 1 on Lap 28 than the Brit. It looked Envision's to lose.
Evans, also got a move on as the laps ticked down. He needed to pressure leader Cassidy into making some kind of error and had the Envision driver within two seconds rather than the three we'd seen up to this point. It felt as if Cassidy did have enough in hand to manage things to the end, however.
Four laps were added on for a 38-lap total but it wouldn't change the result as Cassidy led Evans and Dennis home - with five seconds in-hand.