Defender Shadi Mohamed put the hosts ahead on 10 minutes before Abdallah Shahat doubled the Dervishes' lead four minutes past the half-hour mark.
Ndubuisi Godwin Eze hit Ismaili's third eight minutes from the interval, amid Ahli's inability to redeem themselves.
Ahmed Fathi scored the consolation goal for the Red Devils in the 71st minute, a strike that wasn't nearly enough to ease the away fans' anger.
Few minutes before the final whistle a host of Ahli spectators jeered head coach Hossam Al-Badri, calling for his resignation.
Al-Badri has been under fire for quite a while because of the Egyptian champions' lackluster performances and unsatisfying results on all fronts.
Ahli's loss at Ismaili is their first league defeat this season.
Ismaili were far superior throughout the match and capitalized on their opponents' poor marking on many occasions, firing a good number of long-range shots.
Ahli, who have many players unavailable through injury including Sayed Moawad and Hossam Ashour, suffered a lot of weaknesses.
Their rearguard was stuttering while the presence of holding midfielders Moataz Eno and Hossam Ghaly was hardly felt, allowing Ismaili to tighten their grip on the game.
Frontmen Geddo and Francis Doe barely showed goal menace. Mohamed Ghaddar didn't add any firepower when he replaced the latter striker too.
The six-time African champions, who have two more games in hand, are still in second place with 18 points while Ismaili moved up to seventh spot on 16.
Upper Hand
Ismaili got the upper hand after a calm start to the match, carving out a number of scoring opportunities.
Former Ahli defender Shadi volleyed home a corner kick after Mahmoud Aboul-Saoud failed to palm the ball at full stretch, thanks to his ill-timed jump.
Aboul-Saoud could have been caught napping shortly afterwards when Ahmed Samir Farag released a thunderbolt from a tight angle but the former Mansoura goalie blocked it competently.
In the 22nd minute, Ismaili's Shahat found himself unmarked near the edge of the area. He tried his luck only to see his powerful shot go over the crossbar.
Ismaili ensured their superiority when Ahmed Ahli nodded the ball towards fully-fit Shahat, who beat his marker to the ball and dribbled past Aboul-Saoud to slot into an empty net.
Shortly afterwards, Godwin outran ageing defender Wael Gomaa to once again go one-on-one with Aboul-Saoud and poke home with aplomb.
Ahli star Mohamed Barakat could have reduced the arrears late in the first half but had his effort cleared off the line by an challenger.
Ahli Remain Toothless
The Cairo giants remained toothless after the break, with Al-Badri unable to inject zeal.
He introduced Ghaddar instead of Doe but the Lebanese striker was far from influential.
On the other hand, Ismaili maintained their dominance and free-flowing football.
Ahmed Khairi tried his luck from outside the area, his low effort went inches past the post though.
Godwin could have completed a brace later when he rounded his marker in the area, to canon a shot that struck the woodwork before going wide.
The game's tempo notably slowed down midway through the second period.
Egypt international Fathi pulled one back for Ahli with a pinpoint drive from outside the area on 71 minutes.
Ismaili's Ahmed Al-Gamal saw a low shot narrowly miss the target later on.
Ahli created several scoring chances late in the game but to no avail.