Switzerland's Buemi in his Renault was claiming his fourth race victory out of five in this year's title race, extending his lead in the standings to 15 points over di Grassi.
Audi's di Grassi, who had won the previous round in Mexico, with Buemi only 14th, threatened to hit the front on the 51st and final lap of the famous street circuit, but the title leader held him at bay.
"It is difficult to overtake here and track position is so important," said a rueful di Grassi.
"I had the better car than Seb (Buemi) today but he was in front," he added.
Buemi, who raced in Formula One with Toro Rosso, drove a tactically astute race after claiming the crucial pole position, but admitted he was under real pressure at the finish.
"I did not have much margin and finished on the limit of my energy reserves," he admitted.
"It's the race to win and to start on pole is crucial."
The race was notable for a clash just before halfway involving Jean-Eric Vergne and Nelson Piquet Jr. as the former attempted an overtaking move to claim third spot.
The safety car had to be deployed, with the main beneficiary another ex-F1 racer Nick Heidfeld, who moved up to the final podium spot after starting eighth on the grid.
'Amazing" was Heidfeld's verdict for his barnstorming performance on a circuit where it is notoriously difficult to overtake. He was also making his own bit of history as the first driver to make the podium in the principality in both F1 and Formula E.
He achieved the feat for Williams in F1 in 2005 with a second place finish.
Piquet recovered to take fourth spot, but there was disappointment for Buemi's Renault teammate Nicolas Prost, son of four-time Monaco winner and F1 world champion Alain Prost.
Watched by his father, Prost could only manage 10th spot, but retains third in overall standings.
Supermodel Naomi Campbell was also in the Renault team area to add her touch of celebrity glamor to an event which is held just a fortnight before the F1 race in Monte Carlo and uses most of the same circuit.
Buemi had won the inaugural Formula E race there in 2015, doubling up on that initial success in some style for his French team.
Formula E did not visit Monaco in 2016, but despite the gap Its growing popularity was underlined by packed enclosures and spectator areas on race day.
Formula E, which is highlighting electric-powered racing cars, is in its third season, with a 12-race championship, with the next round in Paris.
It has attracted a number of notable drivers to its ranks, who have to master an extra skill set.
Rather than conventional fueling stops and tire changes as in F1, drivers have to make one compulsory stop to recharge their battery power, which often has to be conserved to reach the finish, with Buemi proving particularly expert in his domination of the new formula.