Envision Racing’s Cassidy was fastest in the first and second sector during the final duel against his fellow Kiwi, putting him more than two tenths up on Evans.
But a blistering final sector ensured Jaguar’s Evans took pole position, and more importantly, denied Cassidy three points in his bid to overthrow Dennis at the head of the standings.
Evans will start sixth for this afternoon's race due to a five-place grid penalty he earned at the previous round in Rome for colliding with Cassidy, which meant neither scored points and they entered the London E-Prix double-header weekend 24 and 44 points respectively behind Dennis.
Cassidy, who will start first alongside Dennis, was handed a spot in the final after Envision Racing team-mate Sebastien Buemi took to the escape road at the Turn 10 chicane, despite being marginally faster in the opening sector.
Andretti Autosport’s Dennis had been eliminated in the earlier semi-final after posting a 1m10.806s which was nearly four tenths slower than Evans, but the championship leader is still set to start second after Evans’ penalty.
Cassidy comfortably beat Porsche’s Pascal Wehrlein by more than half a second in their quarter-final duel, while Buemi progressed through to the same semi-final having set the fastest lap across the entirety of qualifying against NIO 333’s Dan Ticktum, posting a 1m10.404s.
Evans blitzed all three sectors to finish sixth tenths clear of DS Penske’s Stoffel Vandoorne in the first quarter-final duel.
Dennis set a time almost identical to Evans’ best in the same segment of qualifying as he beat McLaren’s Rene Rast by almost four tenths to progress to the semi-final.
Both leading championship protagonists headed their respective qualifying groups, with Dennis fastest in the first session despite suffering a momentary loss of power early in proceedings and a brief brush with the wall on his penultimate effort.
His final lap – 1m10.873s – put him 0.030s clear of Evans as Vandoorne and Rast completed the top four.
Jaguar’s Sam Bird just missed out with his final effort by 0.094s, ahead of Maserati’s Edoardo Mortara and the second DS Penske of Jean-Eric Vergne.
Cassidy had set the fastest time in the first two sectors during the second session but ran wide into the Turn 16 right-hand hairpin on his penultimate run and had to go again.
His final lap, though, was good enough to top the timesheets, a 1m10.721s a tenth-and-a-half clear of team-mate Buemi.
Ticktum and Wehrlein completed the top four, who were covered by less than half a second, with Nissan’s Norman Nato the first driver to miss out on a qualifying duel spot as Briton Jakes Hughes finished ninth in the group.
Formula E London E-Prix Grid:
| Cla | Driver | Car / Engine | Time | Delay |
| 1 | Nick Cassidy | Jaguar | 1'10.604 | |
| 2 | Jake Dennis | Porsche | 1'10.806 | 0.202 |
| 3 | Sébastien Buemi | Jaguar | 1'16.765 | 6.161 |
| 4 | Dan Ticktum | NIO | 1'10.627 | 0.023 |
| 5 | Rene Rast | Nissan | 1'10.893 | 0.289 |
| 6 | Mitch Evans | Jaguar | 1'10.578 | -0.026 |
| 7 | Pascal Wehrlein | Porsche | 1'11.057 | 0.453 |
| 8 | S.Vandoorne | DS | 1'11.071 | 0.467 |
| 9 | Sam Bird | Jaguar | 1'11.078 | 0.474 |
| 10 | Norman Nato | Nissan | 1'11.262 | 0.658 |
| 11 | Edoardo Mortara | Maserati | 1'11.161 | 0.557 |
| 12 | Nico Muller | Mahindra | 1'11.299 | 0.695 |
| 13 | Jean-Eric Vergne | DS | 1'11.275 | 0.671 |
| 14 | Max Guenther | Maserati | 1'11.305 | 0.701 |
| 15 | Andre Lotterer | Porsche | 1'11.333 | 0.729 |
| 16 | Sacha Fenestraz | Nissan | 1'11.343 | 0.739 |
| 17 | A.F.da Costa | Porsche | 1'11.403 | 0.799 |
| 18 | Jake Hughes | Nissan | 1'11.405 | 0.801 |
| 19 | S.Sette Camara | NIO | 1'11.414 | 0.810 |
| 20 | Lucas di Grassi | Mahindra | 1'11.823 | 1.219 |
| 21 | Robin Frijns | Mahindra | 1'11.613 | 1.009 |
| 22 | Roberto Merhi | Mahindra | 1'12.205 | 1.601 |
