The 36-year-old broke the deadlock on the stroke of half-time after tucking away a rebound following Mason Greenwood's effort on goal.
He added a second after 62 minutes, drilling home at the Stretford End before Bruno Fernandes and Jesse Lingard added gloss to a fine 4-1 victory.
Ronaldo was roared on by the home crowd throughout the afternoon, making an instant impact having returned to Old Trafford after 12 years away this summer,
And in an Instagram post shared shortly after full-time, Ronaldo declared himself 'proud' to be back in the Premier League.
'My return to Old Trafford was just a brief reminder of why this stadium is known as the Theatre of Dreams,' he wrote.
'For me, it has always been a magical place where you can achieve everything you set your mind into.
'Along with all my teammates and with the amazing support that we always get from the stands, we face the road ahead with confidence and optimism that we'll all be celebrating together in the end.
'Proud to be back at Man. United and playing in the Premier League once again, but above all, happy to help the team!
'Let's go, Devils!'
Victory over Steve Bruce's side temporarily moves United to the top of the Premier League table – although Chelsea, Liverpool and Everton could join them on 10 points with wins this weekend.
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