Despite Moemen Zakareya first goal in the first half Al-Ahly still played under intense pressure to protect their unbeaten run in the Egyptian top flight.
But introduction of former 36 Lion striker seem to bring succour as he ensured victory as he head home a second in stoppage time and spark wild celebrations among the hundreds of fans who were allowed to attend the match.
The victory extends the lead of leaders Ahly over perennial rivals Zamalek, who are third, to 11 points but the latter still have two matches in hand.
Ajayi had only been on the pitch for less than three minutes when he rose unmarked to head home a left wing cross from Fathi. Meanwhile, Zamalek's attacking plans were disrupted after Nigerian forward Stanley Ohawuchi pulled out during the warm-up after seemingly aggravating an ankle injury.