One of United's greatest ever players returned in style to the delight of home supporters, although Newcastle goalkeeper Freddie Woodman was complicit in both goals.
Ronaldo boosted his United tally to 120 as he pounced on a first-half error and then blasted straight through Woodman after the break, setting Ole Gunnar Solskjaer's men on top of the Premier League table.
It was a good day….what an atmosphere @cpfc …now we prepare for next week 👏❤️💙🦅— Osian Roberts (@Osian_Roberts) September 11, 2021
Newcastle had equalised through Javier Manquillo between those strikes but otherwise lacked the cutting edge that Ronaldo displayed, which was also in evidence when Bruno Fernandes and Jesse Lingard wrapped up the points late on.
United was predictably dominant from the outset and Raphael Varane headed across the face of goal, yet it was given a scare when Joe Willock stole possession from Fernandes in the home area, only to blaze over with team-mates up in support.
The moment a raucous Old Trafford crowd had been waiting for arrived on the stroke of half-time as a low Mason Greenwood strike escaped Woodman's clutches and fell kindly for Ronaldo to slam home.
Newcastle had been growing into the game and did not allow that setback to dampen its spirits, quickly levelling in the second half when Miguel Almiron led a counter-attack and Allan Saint-Maximin slipped in Manquillo to finish.
But a United break proved every bit as efficient, with Luke Shaw carrying the ball forward and feeding Ronaldo to fire between Woodman's legs.
David de Gea stood tall to block from Joelinton when Newcastle threatened another equaliser, but sublime finishes from Fernandes from around 25 yards and Lingard, who skipped past Jamal Lewis in the box, made the game safe amid a party atmosphere.