The New Zealander had scored only one point from the first six races of 2024/25, but this win - from lights-to-flag - saw him bag a hat-trick of victories.
That also made it five wins in six for Jaguar TCS Racing on the weekend both he and ever-present Team Principal James Barclay leave the outfit, the latter after overseeing the brand's motorsport return and all 127 of their Formula E races to-date.
RESULTS: The full classification for the 2025 Marvel Fantastic Four London E-Prix Round 16
Cassidy crossed the line 13.5 seconds ahead of Mahindra Racing's Nyck de Vries for the second-largest winning margin in Formula E history - only Antonio Felix da Costa had won three on the spin before. Another second spot for de Vries helped Mahindra seal a best Teams' finish of fourth since 2017/18.
Sebastien Buemi (Envision Racing) followed up a Jakarta podium with third here - up from 19th on the grid at the outset. Jake Dennis (Andretti) came home fourth, with Mitch Evans (Jaguar TCS Racing) following in fifth - a five-second penalty for speeding under Full Course Yellow conditions costing him the second place he'd secured on-track.
Both Nissans had to finish in the top four to have a chance at beating Porsche to the Teams' and Manufactuers' titles but it wasn't to be as champion Oliver Rowland looked to lead the charge. Contact with Nico Mueller on Lap 16 ended his and the Andretti driver's races. Even so, Rowland was on-hand to be crowned Drivers' World Champion and soak up the adulation of a sold out home crowd - having taken the honours back in Berlin.
That meant Porsche secured maiden Teams' and Manufacturers' World Championship titles, with Jaguar TCS Racing also beating Nissan to second in both the Teams' and Manufacturers' running at the last.
As it happened...
Cassidy sliced across to lead Guenther, Evans, de Vries, Wehrlein and Ticktum away from pole without incident this time around at the Excel - that was until Edoardo Mortara found himself out of the running losing drive from a promising eighth on the grid.
On Lap 3 into the final sector Mitch Evans made a move around the outside of Guenther to take P2 and make it a Jaguar one-two as it stood. Guenther was then shuffled further back at Turn 16 a tour later with de Vries and Wehrlein jumping the DS driver. Rowland followed through Sector 1 for fifth beyond the German - forceful but fair from the champion.
Wehrlein and Rowland were the first of the lead six to jump for their initial mandatory 50kW, four-wheel drive ATTACK MODE boost - seeing the Porsche driver pushed to seventh while Rowland retained fourth before climbing to third over the start/finish on Lap 9.
On Lap 10, Rowland tried to make a move stick around the outside of de Vries over the bump around the back of the circuit. Second place was the prize but contact left Rowland's car damaged after a staunch defence by de Vries. The The champion did find his way by a few turns later but not unscathed.
On Lap 12, de Vries jumped for ATTACK allowing Evans through into third, with his Jaguar teammate and race leader Cassidy building a three-second margin to Rowland sandwiched in-between the pair.
Cassidy led Rowland, Evans, de Vries, Wehrlein, Guenther, Mueller, Dennis, Vergne and Bird on Lap 13 - before de Vries jumped Evans in ATTACK at the chicane for third spot. The Season 7 champion then made it beyond Rowland into Turn 1 on Lap 14 - despite the best efforts of Rowland in defence, but crucially 50kW down, without ATTACK versus the Mahindra driver.
Nico Mueller made moves - beyond Evans for fourth in ATTACK, up from 14th on the grid. However, into Turn 1 on Lap 16 his progress, and that of Rowland, would come to a swift end as the pair clashed. Rowland was left in the Turn 2 wall and out, while Mueller suffered race ending damage - both enforcing a Safety Car.
That would prove crucial for the Teams' and Manufacturers' battle - all but handing the trophies to Porsche.
The race went green again on Lap 20 with Cassidy leading Evans, de Vries, Wehrlein, Guenther, Dennis, Buemi, Frijns, Bird and Vergne - however, Evans had a five-second penalty applied for speeding under Full Course Yellow conditions early on.
A hat-trick of wins looked to be on for Cassidy, with a 1.5 second lead in-hand on-track to his teammate, as well as three percent of usable energy in his pocket - allowing him to really push for the final six laps of Season 11.
From there, he was able to draw out a stunning 13.5-second margin to head home de Vries, after Evans' five-second penalty.
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