Portsmouth had no answer to all his tricks and skill as he tore them apart with two goals in the opening 13 minutes.
He was on the end of a glorious move which put United ahead in the tenth minute.
Paul Scholes, making his first start since the end of October, found Ronaldo with one of his trademark passes.
Ronaldo chested the ball down for Nani, who returned the pass to his compatriot on the left side of the Pompey box.
Ronaldo coolly took the ball in his stride before stroking it left footed past Pompey goalkeeper David James and into the net from the corner of the six-yard box.
But if Ronaldo's opening goal was sublime, his second was simply magnificent. Sol Campbell brought him down 25 yards from goal and Ronaldo stepped up to hit a magnificent swerving, dipping free-kick which crashed into the top right-hand corner of the net.
The shot gave James no chance and even the England goalkeeper was left with a rueful smile on his face at Ronaldo's brilliance.
The stage seemed set for United to run up a cricket score, but a combination of their profligacy in front of goal and superb goalkeeping by James kept the score down.
Wes Brown, Ji-Sung Park, Wayne Rooney and Nani all missed chances to extend the lead in the first half.
Six minutes after the interval, James made the first of several fine saves when his reflexes kept out Rio Ferdinand's header from Nani's corner.
Ronaldo fluffed the chance to record his second hat-trick for the club with a weak shot from 15 yards after Rooney's excellent tackle on Campbell had created the chance.
James twice saved Pompey in the 73rd minute. Sean Davis directed a poor header towards Rooney just inside the Pompey half and he worked the ball on to Ronaldo, whose pass set up Park.
But James hurled himself at the South Korean's feet to deny Park and then produced a brilliant stop to keep out Nani's effort from point-blank range.
Seconds later, James clawed away a fierce drive by Carlos Tevez from the right side of the Pompey penalty area and Anderson was inches wide of the far post with his shot after Pompey failed to clear the danger.