Ronaldo scored in the 42nd, and added two more in the 79th and 81st minutes to seal the match after Niklas Moisander’s 56th minute header had made it 2-1 and briefly given Ajax hope.
But Ronaldo took his Champions League career tally to 42 with two late strikes for a morale-boosting win for Madrid ahead of this weekend’s league clash with archrival Barcelona.
Benzema’s 48th minute strike was the highlight. The France striker hit a powerful overhead volley that left Ajax goalkeeper Kenneth Vermeer clutching at thin air.
The win sent Madrid to the top of Group D and left Ajax pointless in its first two European outings this season.
The two sides had met in the past two Champions Leagues with Madrid winning all four matches without conceding a goal to four-time European champion Ajax.
Madrid dominated the first half and should have sealed the match by the break. It was only Ajax goalkeeper Vermeer who kept his side in the hunt with a string of good saves to deny Ronaldo, Kaka and Benzema.
But Vermeer was helpless when Benzema cut in from the left in the 42nd minute and pulled the ball back through a crowd of Ajax defenders for Ronaldo to tap in from close range.
The closest Ajax came to scoring in the first half was when Madrid goalkeeper Iker Casillas dropped an innocuous corner by Eriksen, only for right back Ricardo van Rhijn to head the sharp chance over from close range.
Ajax was left reeling by Benzema’s goal, but Moisander’s header gave Ajax a confidence it had lacked in the first half and Frank de Boer’s team began to exert some pressure on Madrid.
Even so, it was Madrid that kept creating chances, with Benzema heading over from close range and Vermeer saving a low shot by substitute Angel di Maria.
And it was Ronaldo who made the pressure pay, curling in a right-foot shot in the 79th minute and grabbing his hat trick two minutes later with a deft chip.