Cassidy, driving for Envision Virgin Racing, which counts three wins on the Red Hook circuit, was only the third driver to take to the track in the top-six superpole shootout.
With three fastest sectors, his 1m09.336s effort sealed provisional pole by some 0.2s and he survived the advanced of champions Vergne and Sebastien Buemi to land a second pole after Rome – when the Super GT and Super Formula champion span out on the opening lap.
DS Techeetah driver Vergne had eclipsed Cassidy through the middle sector but for the 0.1s he gained, he lost that time again in both the first and final sectors to land second place.
He ran 0.161s adrift of Kiwi Cassidy but beat first superpole runner Alex Lynn to the front row, beating the impressive effort of the Mahindra Racing driver by 0.039s.
Lynn, who scored pole in New York City on his series debut back in 2017, will start alongside BMW Andretti’s Maximillian Guenther as Buemi will line up fifth.
With a new gearbox, motor and invertor aboard his Nissan e.dams machine and carrying the advantage of a group four running order, Buemi’s tepid season seemed to turn a corner when he topped the group stages.
But the 2015-16 champion suffered a heavy lock up into Turn 1 to immediately stymy his final flying lap and he ran to fifth ahead of a subdued effort for Pascal Wehrlein.
The Porsche driver, disqualified from his and the manufacturer’s maiden win in Puebla, completed the fastest final sector of the day but ran 0.414s off Cassidy’s pace.
Lucas di Grassi, confident the bumpy New York City circuit mimics the nature of the Rome track on which he led prior to a driveshaft failure, was the first to miss superpole.
The Audi driver, who led a team 1-2 in the Puebla opener last time out, was pipped to the top-six cut-off by Lynn by just 0.013s but beat a recovering Oliver Rowland’s second effort.
Rowland was the cause of a bizarre red flag incident during the second group phase.
The Nissan e.dams charge entered Turn 1 of his flying lap but clipped the tall inside kerb at the apex, which sent his car into a spin, with the rear then collecting Sam Bird.
Bird, who was able to take part in the session after a mammoth tub build for Jaguar Racing following an uncharacteristic error in free practice, admitted his car balance was wayward.
Edoardo Mortara, Venturi Racing, Silver Arrow 02
Photo by: Andrew Ferraro / Motorsport Images
That turned his qualifying entry into an effective extra shakedown period, meaning he slowed at the exit of the opening right-hander to give Rowland space on his flier.
But as Rowland rotated, the rear of his car flicked round and pincered Bird by the wall.
The session was eventually resumed with all six cars able to take part, and double Mexico front-row starter Rowland snared eighth ahead of Alexander Sims and Andre Lotterer.
Another major headline was created by Edoardo Mortara, who currently leads the standings by a comparatively lofty 10 points ahead of the final three double-headers of the season.
The Venturi Racing driver knocked the scenario switch on his steering wheel, which took his Mercedes customer car out of the 250kW full power mode and into a lower 200kW setting.
That cost him 1.63s at the time and puts him last on the grid, having fallen foul of a similar issue that effected the work Mercedes car of Nyck de Vries in Monaco qualifying in May.
This incident gave his title rivals breathing space in the unfavoured group one battle on a comparatively green surface, with current runner-up Robin Frijns edging out his rivals.
The Envision Virgin Racing driver ran to 11th ahead of Dragon Penske Autosport’s group four entrant Sergio Sette Camara and then defending champion Antonio Felix da Costa.
Behind Oliver Turvey and BMW Andretti rookie Jake Dennis, the Jaguar machine of Mitch Evans nabbed 16th with the second NIO 333 of Tom Blomqvist set to start in 17th.
Another stunted session from Mercedes put de Vries just 18th ahead of Norman Nato and a subdued Bird, while Stoffel Vandoorne’s shot at pole was ruined by a slow last sector.
The Silver Arrows 02 driver, running in group two, was in contention with the fastest middle sector at the time before he dropped a hefty 1.3s in the final sector to qualifying 21st.
Rene Rast, Audi’s full-season rookie, was another to lose out in the title race. Currently fourth in the drivers’ table and only two points off second, the German was 1.5s adrift of the group qualifying pace and will start down in 23rd.