The Yorkshireman found a bundle of time in the middle part of the lap to flex a huge advantage over di Grassi, which gave him enough of a buffer when he went wide at the final corner.
Although di Grassi had started the lap in better fashion, Rowland got into his stride and found a quicker line through the stadium section to crack open the lead, and was much more committed through the following sweepers to leave the Venturi driver unable to find a way to recover.
Rowland was clearly in the groove in the final half of the lap and had found 1.5s over Pascal Wehrlein over the last two sectors in their duel, crossing the line with a mammoth 1.637s advantage despite a side-to-side slide on the exit of Turn 19.
Wehrlein seemed to be considerably more tentative in the closing stages of the lap, setting Rowland up for a battle with his future team-mate.
Di Grassi had earlier denied Mitch Evans a finals appearance, with the Brazilian holding a 0.4s advantage by the close of the second sector, but a slightly more wayward final part of the lap gave Evans a glimmer of hope. The Jaguar driver just fell short, losing out by just under 0.2s.
Only the drivers from Group B, armed with the knowledge of the grippier parts of the Seoul circuit having faced worse conditions during their initial qualifying phase, progressed into the semi-finals.
Evans and Edoardo Mortara were drawn in a box-office duel of those still within the shot of a title tilt. After being cleared of transgressing the rules on minimum pitstop time, Evans acquitted himself well within the damp conditions, finding a clear 1.1s advantage over the Swiss driver.
Di Grassi atoned for his final loss against Jake Dennis in London by beating the Avalanche Andretti driver in their Seoul quarter-final with a clear advantage of around 0.7s, despite the British driver's intermittent periods where he looked to be catching his rival.
Sergio Sette Camara made another duels appearance, but his Dragon Penske machinery came to a stop in the stadium section and thus granted Rowland easy passage through to the semi-finals.
Points leader Stoffel Vandoorne was dumped out of his quarter-final with Wehrlein, as the Porsche driver held his nerve despite a late slide to progress as track conditions became worse.
Wehrlein had nearly 0.9s in hand over Vandoorne but, after a near slip at Turn 19, shipped more than half a second to Vandoorne - but held enough of an advantage to book a semi-final with Rowland.
The early onset of rain in the Group A session had put Vandoorne at risk, but the points leader kept his place in the top four as nobody was able to improve as the track became slippery within the final stages of the session.
As the first flurry of qualifying laps came in, drizzle began to emerge overhead - but Mortara stayed out after his initial run and prevailed through the early precipitation and improved up to first.
Vandoorne instead had elected to pit and returned to the circuit on fresh tyres for the second half of the 12-minute session, but found the surface more slippery and could not improve on fourth.
But neither Antonio Felix da Costa or Robin Frijns could keep enough control to string a quicker lap together, sparing Vandoorne's blushes.
Da Costa and Frijns were hence denied a chance to make the duels, as Andre Lotterer endured an off-colour session and was last in the Group A order.
Jean-Eric Vergne was dumped out of the top four of Group B, as the confusion over Evans' pitstop time briefly looked to give the Frenchman a reprieve.
The sonorous clap of thunder pervaded the start of Group B's qualifying session as the rain dampened the circuit in the brief interval between groups.
Di Grassi ended the first set of runs with the best time, but was deposed amid an immediate set of second runs, which eventually resulted in Wehrlein making it to the top with a 1m31.202s.
The session was red-flagged after Mahindra's Alexander Sims hit the wall, but the Briton extricated his car from the barrier and limped back to the pitlane with just over six minutes left on the clock.
Once the session restarted, the track conditions looked to be improving and the timing boards resembled a slot machine, but a monster pair of laps for both Wehrlein and Rowland booked their places into the top four.
Di Grassi nipped into third, as Vergne was on the bubble in fourth before being displaced by Evans. The Kiwi's time was initially removed from the board after apparently transgressing the minimum pitstop time, but this had occurred during the red flag and as such his time was reinstated.
This denies Vergne the chance to stay in mathematical contention for the title, as he needed pole to remain within the boundary.
Seoul E-Prix - Qualifying results:
|1||30||Oliver Rowland||Mahindra Racing|
|2||11||Lucas di Grassi||Venturi|
|3||9||Mitch Evans||Jaguar Racing|
|4||94||Pascal Wehrlein||Porsche Team|
|5||27||Jake Dennis||Andretti Autosport|
|8||7||Sergio Sette Camara||Dragon Racing|
|11||10||Norman Nato||Jaguar Racing|
|12||13||Antonio Felix da Costa||Techeetah|
|13||33||Dan Ticktum||NIO Formula E Team|
|14||99||Antonio Giovinazzi||Dragon Racing|
|15||17||Nyck de Vries||Mercedes|
|16||4||Robin Frijns||Virgin Racing|
|18||3||Oliver Turvey||NIO Formula E Team|
|19||37||Nick Cassidy||Virgin Racing|
|20||36||Andre Lotterer||Porsche Team|
|21||29||Alexander Sims||Mahindra Racing|
|22||28||Oliver Askew||Andretti Autosport|
|View full results|