Mohsen Abou-Greisha's 49th minute header earned Ismaily full marks and pushed them to second place on 38 points, two points behind Ahly, who also have a game in hand.
Ahly, who are looking to win a fifth consecutive league title, could at least have drawn level later in the second half but Angolan striker Flavio failed to convert a penalty kick.
It was also a sweet revenge for Ismaily, who lost to the Cairo Red Devils by the same scoreline in the first round of the championship.
Ahly were without several players due to injury and gave Palestinian goalkeeper Ramzi Saleh his league debut ahead of Amir Abdul-Hamid, who was to blame in the 2-2 draw with Petrojet in their previous game.
Samm Audu, Goal.com