Visitors Al Ain came into the clash on the back of an eight-match unbeaten run, but that sequence looked in grave danger until Ahmed struck four minutes from time.
Al Shabab had earlier taken the lead midway through the second half courtesy of Edgar Silva's penalty.
But, with time running out, Miroslav Stoch teed up Ahmad, who extended Al Ain's undefeated run, and the home side saw Hassan Ibrahim dismissed for a second bookable offence a minute before the end.
Al Ain are now level on points with leaders Al Jazira, with Al Shabab just one point back.
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