On his team's home turf, Andretti driver Dennis produced a lap of 1m08.931s in the pole shootout to beat Nissan rival Fenestraz by a scant 0.079s.
It gives him three bonus points that put him two points clear of Pascal Wehrlein at the head of the standings heading into Saturday's race.
Fenestraz had the better start to the lap of the two finalists, having been half a tenth up after sector two, but a poor final sector allowed Dennis to claw back the lost ground and take pole for the first time since last year's London E-Prix.
Making up the second row of the grid will be Fenestraz's Nissan team-mate Norman Nato and McLaren's Rene Rast.
Having dispatched Maximilian Gunther in the quarter-finals, Fenestraz was up against Nato in the first of the semi-finals, but eased through to the final with the first sub-1m09s effort of the weekend, a 1m08.920s, beating Nato by 0.344s.
In the second semi, Dennis eclipsed Fenestraz's benchmark by a single thousandth of a second to beat Rast by a comfortable 0.455s and book his third consecutive final appearance.
Earlier on, Dennis beat the other McLaren of Jake Hughes in the quarter-finals to become the only non-Nissan powertrain user to reach the last four.
Maserati MSG man Gunther will line up fifth ahead of Hughes and Porsche's Antonio Felix da Costa, who lost out to Rast in his quarter-final.
The other driver to make it out the group stages was Jean-Eric Vergne, but he and his DS Penske team-mate Stoffel Vandoorne will both start Saturday's race from the pitlane.
That was after DS Penske was found using illegal equipment in the pitlane in practice to scan the barcodes of the tyres of every car in the field, something stewards felt could give the team a major advantage.
Dennis was the only driver in the top four of the standings to make into the duels, with Porsche driver Wehrlein set to start 18th after ending up only 10th-fastest of the 11 drivers in the opening group.
Nick Cassidy, third in the points, was only sixth-fastest for Envision, albeit best of the four Jaguar-powered cars in the field.
Cassidy is due to start from 10th behind the second Andretti car of Andre Lotterer, back in FE action after skipping the previous round at Jakarta, and Abt man Robin Frijns.
Neither of the works Jaguars made it out of the second group, with Sam Bird ninth-fastest and outside title contender Mitch Evans failing to set a lap at all after the team elected to change his battery and powertrain following an issue in second practice.
The Portland E-Prix is due to get underway at 5pm local time.
Portland E-Prix - qualifying results:
| Cla | # | Driver | Team |
|---|---|---|---|
| 1 | 27 |
 Jake Dennis |
 Andretti Autosport |
| 2 | 23 |
 Sacha Fenestraz |
 Nissan e.dams |
| 3 | 17 |
Norman Nato |
Nissan e.dams |
| 4 | 58 |
 René Rast |
McLaren |
| 5 | 7 |
Maximilian Gunther |
 Maserati Racing |
| 6 | 5 |
Jake Hughes |
McLaren |
| 7 | 13 |
 Antonio Felix da Costa |
Porsche Team |
| 8 | 36 |
Andre Lotterer |
Andretti Autosport |
| 9 | 4 |
 Robin Frijns |
Team Abt |
| 10 | 37 |
 Nick Cassidy |
Envision Racing |
| 11 | 48 |
 Edoardo Mortara |
Maserati Racing |
| 12 | 51 |
Nico Müller |
Team Abt |
| 13 | 11 |
 Lucas di Grassi |
 Mahindra Racing |
| 14 | 33 |
Dan Ticktum |
NIO Formula E Team |
| 15 | 10 |
Sam Bird |
Jaguar Racing |
| 16 | 16 |
Sébastien Buemi |
Envision Racing |
| 17 | 8 |
 Roberto Merhi |
Mahindra Racing |
| 18 | 94 |
Pascal Wehrlein |
Porsche Team |
| 19 | 3 |
Sergio Sette Camara |
NIO Formula E Team |
| 20 | 9 |
Mitch Evans |
Jaguar Racing |
| 21 | 25 |
Jean-Eric Vergne |
DS Penske |
| 22 | 1 |
 Stoffel Vandoorne |
DS Penske |
| View full results | |||
