Newcastle, who battled hard and showed character in the first half when they had a perfectly good goal by Michael Owen ruled out for offside when he was just on, fell apart in the second period when United hit all six of their efforts.
Whoever replaces Sam Allardyce at St James' Park has a monumental task to turn around the fortunes of this great club.
Had Newcastle led at half-time, it would have been totally against the run of play as only a brilliant one-handed save by Shay Given from Wayne Rooney and a goalline clearance by Claudio Cacapa from John O'Shea's header kept the home side out.
The second half produced an onslaught which left Newcastle dead and buried.
It started within the first minute of the second period when Steven Taylor cleared shots by Carlos Tevez and Rooney off his line within seconds of each other.
But United had only until the 49th minute to wait before breaking the deadlock.
Newcastle skipper and former United favourite Alan Smith was harshly adjudged to have tapped the back of Ronaldo's ankle a couple of yards to the left of the penalty area.
Up stepped Ronaldo to drill his low right-foot free-kick under the Newcastle wall and into the net beyond Given.
Five minutes later United were 2-0 ahead and the goal stemmed from a dreadful Newcastle blunder.
Jose Enrique's back pass to Given saw the Newcastle goalkeeper kick the ball straight to Cacapa, standing 12 yards from goal.
It rebounded to Ryan Giggs, who controlled it and passed across the box to his right for Tevez to score with a side-foot shot from ten yards.
United's third goal, in the 70th minute, was arguably the best of the bunch.
Michael Carrick found Rooney, who neatly laid the ball on for Tevez and the Argentinian threaded a glorious pass through to Ronaldo, who buried his right-foot shot into the corner of the net from ten yards.
Newcastle were now crumbling, but they collapsed completely in the last five minutes.
In the 85th minute, Ronaldo played a short corner to O'Shea on the left corner of the Newcastle penalty area.
He played the ball on to Rooney, whose clever pass saw Rio Ferdinand racing free on the right side of the box to score with a shot from eight yards.
Ronaldo completed his hat-trick in the 88th minute.
O'Shea's ball from the left corner of the Newcastle box was not cleared by Enrique and Ronaldo pounced to beat Capapa and Enrique to score with a shot from ten yards.
Goal number six came deep into injury time.
O'Shea's cross from the left was headed on by substitute Nani and Tevez stretched to hit the ball in from eight yards on the far post, with the ball going in off the underside of the bar.
Newcastle complained that the ball had not crossed the line and Smith was sent off by referee Rob Styles for taking his protest too far.