Ismaili looked as though they would claim a comfortable victory when two early goals from Abdullah Al-Shahat and Ibrahim Yehia gave them a commanding lead.
Towering striker Ramie Rabie narrowed the gap 11 minutes before the break after capitalizing on a dreadful error by Ismaili keeper Mohamed Sobhi.
Ismaili's combative defender Sherif Abdul-Fadil was sent off six minutes later for a second bookable offense as Contractors' hopes of grabbing the equalizer were revived.
The hosts made the most of their numerical advantage. Two second-half goals from Samuel Kyere and Ehab Al-Masri gave the Green Mountain Wolves their second win of the season.
Last season's runners-up Ismaili remain second in the league table with 10 points.
Leaders Petrojet, who have a better goal difference than Ismaili, have a chance to go three points clear on the top if they beat Masri later on Sunday.
Contractors are fifth on nine points. Ahli, Zamalek and Harras Al-Hodoud will not be in action this round due to their African commitments.
Blistering Start
Ismaili's blistering start saw them score two goals in the space of nine minutes.
Right-back Al-Shahat opened the scoring after six minutes when his left-footed shot from the edge of the area went in at the near post.
Defender Ibrahim Yehia extended Ismaili's advantage three minutes later when he took advantage of some sloppy defending and a slow reaction from Contractors keeper Mohamed Al-Akabawi.
He was found unmarked by a left-wing cross to side-foot home past Al-Akabawi, who could have saved Yehia's effort had he reacted quicker.
Abdul-Fadil was booked for bringing down a Contractors striker inside the area as the referee pointed to the spot in the 31st minute.
Rabie stepped up but his weak effort was easily collected by Ismaili guardian Sobhi.
But the former Ahli striker made amends for the wasted penalty shortly after.
He narrowed the gap in the 34th minute when he headed into an empty net after Sobhi parried a harmless cross into his path.
Turning Point
Abdul-Fadil was sent off after receiving a second yellow card for committing a needless foul. His dismissal helped Contractors take a full control of the game.
Ghanaian midfielder Kyere drew Contractors level two minutes past the hour mark after some good work by substitute Khaled Yosri.
Yosri provided a precise low cross for the unmarked Kyere, who drove a shot that found its way past the helpless Sobhi.
Yosri turned from provider to scorer in the 64th minute as the home fans erupted into ecstasy.
Rabie penetrated down the right flank, cut inside and sent a low cross that eluded Sobhi, leaving Yosri with the easiest of tap-ins.