"We had too many individuals in the first half trying to walk the ball into the net," said United manager Sir Alex Ferguson. "But the second-half display was marvelous, absolutely fantastic and could not be faulted."
Ronaldo's three goals increased his tally to 22 this season.
"To score 22 goals at this stage of the season for a wide player is fantastic, incredible," added Ferguson. "But I am not surprised. He is always practicing, improving and his decision-making is getting better."
Five minutes after halftime, Ronaldo opened the scoring with a free kick that he struck under the Newcastle wall as the Magpies jumped. His second goal followed a good move involving Wayne Rooney and Carlos Tevez, and his third late in the game again gave Irish keeper Shay Given no chance.
Tevez added two goals himself, and Rio Ferdinand scored the other as United took advantage of some awful play with and without the ball by Newcastle's defenders.
The game marked the debut of interim manager Nigel Pearson following the departure of Sam Allardyce on Wednesday.
"It's a very, very difficult day for us, it's very painful," he said. "They were a lot better than us and the way we folded at the end, it is difficult to take. We as a club need to move forward quickly to get the situation resolved."
Newcastle's efforts to pry Harry Redknapp away from Portsmouth appeared to prove unsuccessful as Pompey said no to Redknapp's proposed move north.