Madrid attacked from the first whistle and the home side had goalkeeper Kenneth Vermeer to thank for the scores remaining level.
The hosts rarely threatened and it was Los Blancos who again came close midway through the half as Kaka's deflected drive was tipped away by Vermeer.
However, Madrid's pressure paid off just minutes before the half-time whistle when Ronaldo prodded home Benzema's cross.
They second half was only three minutes old when they doubled their lead in spectacular style as Benzema planted a superb overhead kick past Vermeer.
With Jose Mourinho's side seemingly in control, Ajax cut the deficit almost instantly when Niklas Moisander headed in Christian Eriksen's corner.
The Dutch side should have levelled the contest with 20 minutes to play but Danny Hoesen blasted over from close range with his first touch of the ball.
Instead, Madrid put the outcome beyond doubt when Ronaldo completed his second hat-trick in as many games.
The Portuguese combined well with Benzema on 79 minutes to curl the ball round Vermeer before getting his third thanks to a delightful chip over the goalkeeper.