His 31st goal of the season in the 76th minute was cruel on the home side who at last produced a performance their long suffering fans could cheer.
They recovered from an early pummeling to give the title holders a real scare and, but for Ben Foster, would have taken the lead.
The goalkeeper made two excellent saves on his debut to deny Kenny Miller and United's wastefulness in front of goal threatened to prove costly until Ronaldo finally made one count.
But at least Derby will be encouraged by the way they bounced back from their 6-1 thrashing at Chelsea in midweek to give United a searching examination.
Sir Alex Ferguson's team knew they had been in a game and the way they celebrated the goal showed how relieved they were to escape an embarrassing result against the Premiership's bottom club.
It should have been so much easier for the visitors who missed a string of early chances as Ronaldo and Wayne Rooney cut through Derby's defence.
Ryan Giggs twice blazed over, Ronaldo struck a post and brought a fine save out of Roy Carroll with a curling right-foot shot in the 33rd minute.
But Derby came back strongly and, in the 38th minute, Eddie Lewis got the better of John O'Shea to cross from the left and Miller's low shot was pushed away by Foster.
A minute later, Miller skipped past Wes Brown but Foster plunged low to his left to keep out the Scot's drive. It was a fine save but Miller should have scored.
Derby kept snapping away at United in the second half but the best chances still fell to the visitors.
Ronaldo had a 35-yard free-kick parried by Carroll and he held his head in frustration when he dragged another shot wide of the right-hand post.
The home fans were starting to dream of a draw or even better when United at last broke through.
Dean Leacock had been superb but his clearance with 14 minutes left was quickly played back in and Rooney reacted first to reach the ball on Derby's left.
He got past Jay McEveley and delivered a cross which Ronaldo placed past Carroll's right hand and into the bottom corner of the net.
Derby never gave up but Ronaldo could have scored again near the end although that would have been hard on Paul Jewell's team who gave one of their best performances of the season.