The central defender has penned a four-year deal with the Red Devils, the club said on their official website.
Zamalek looked close to signing Abdul-Fadil after having an EGP7-million bid for him accepted by Ismaili.
However, the White Knights were unable to meet his contract demands and the deal eventually collapsed.
Ismaili's fierce rivals Ahli were waiting in the wings to acquire Abdul-Fadil's services.
"Ahli increased their offer to EGP7.5 million and agreed to sell right-back Ahmed Sedik to us as part of the deal," Ismaili chairman Nasr Aboul-Hassan told FilGoal.com.
"We had to accept Ahli's offer because, unlike Zamalek, they will pay the transfer fees in cash. Ismaili are pushed for money."
Although Ahli's current squad boasts several ex-Ismaili players, none of them joined Ahli directly from the Dervishes.
Abdul-Fadil has become the second player Ismaili sell to Ahli in a decade after midfielder Mohamed Abdullah, who joined the six-time African champions in 2005.
The heated rivalry between both sides makes it difficult for them to swap players.
Abdul-Fadil's arrival will boost Ahli manager Hossam Al-Badri, who has seen his side suffer from defensive problems during their pre-season friendlies.
The 26-year-old, who can also play as a right back, is likely to partner Egyptian international Wael Gomaa in the heart of Ahli's defense.
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