The Porsche driver took a commanding lead early within the race on lap 21 after a brief safety automotive break that cleared debris, and he conserved his remaining energy over the remaining six laps to secure his second victory of the weekend at Portland International Raceway.
After da Costa won the race on Saturday after away race winner Mitch Evans was handed a five-second penalty, there was little question about da Costa's victory on Sunday, despite late pressure from Robin Frijns.
Evans finished on the rostrum to inside 12 points of Jaguar team-mate Cassidy, who, after losing the lead in Saturday's race, didn’t rating a degree on Sunday when he pitted for a front wing substitute.
Another hero of the race, Pascal Wehrlein, finished fourth, excluding his front wingman, following earlier contact with Edoardo Mortara, and is now 12 points behind Cassidy in the general standings.
The Porsche driver's front wing became lodged under his automotive after colliding with Mortara, then flew straight into the trail of Sam Bird, leaving the McLaren driver on the grass along the back straight – an incident noted by the stewards.
However, with bodywork damage behind him, Wehrlein continued racing and scored small print within the fight for his first Formula E title.
His advantage over Cassidy was reduced when the Jaguar driver was forced to pit following a collision at Turn 1 on lap 13 involving defending champions Jake Dennis and Caio Collet, who all suffered front wing damage within the collision.
Da Costa also suffered front wing damage on lap 15 after colliding with Mortara at Turn 10 at the top of the straight, which caused the Mahindra's left rear tyre to puncture and sent Mortara off the track.
The incident was investigated by flight attendants, but no further motion was taken. Da Costa continued to destroy the plane.
With debris strewn in regards to the track, the protection automotive finally got here out on lap 18, saving drivers like Cassidy and Dennis who managed to meet up with the remaining of the sphere.
With just two laps left within the race, Frijns was within the lead, followed by da Costa, Wehrlein and pole-sitter Jean-Eric Vergne.
Evans finished fifth, but with one other attack mode to make use of he will surely have been at a drawback because the pace picked up considerably once the race resumed.
Da Costa took a right away lead, which he maintained through the primary corner of the restart, before Evans made a second activation and dropped to seventh before breaking through the pack again.
On lap 23 he demoted Wehrlein for third place at the primary corner and repeated the move at Frijns a lap later, profiting from the additional power.
However, with fuel running low, he was unable to challenge da Costa within the remaining laps and fell to Frijns. The Envision driver moved into second at Turn 1 on the penultimate lap and secured two more podiums in America this weekend.
Despite Cassidy having significantly more energy than the drivers ahead of him, the faster pace of the race following the protection automotive meant he was unable to capitalise on his advantage and only moved as much as thirteenth.
It means Cassidy leaves the doubleheader in Portland with no points and a significantly reduced lead, with just two races remaining in London.
