Wayne Rooney provided the icing on the cake for United with a second goal two minutes from time after Ronaldo's shot had been blocked.
But it was not one of United's best performances against a Villa side that battled hard only to suffer their first defeat in six games.
United, however, marched to their 13th successive win over Villa with Ronaldo popping up for the vital 81st-minute goal when the game appeared to be heading for a replay.
Typically, United finished with a flourish as substitute Rooney hit the high notes in the final 20 minutes and grabbed a smartly taken 89th-minute goal.
Strangely, United were off-key for long periods of the game and made hard work of the win. Skipper Ryan Giggs had two good opportunities in the first half but his misses enabled Villa to remain in the game.
The interesting side-issue of the tie was the attendance of England's new coach, Fabio Capello, who must have had mixed feelings about some of the individual performances.
Ashley Young opened on a bright note but, to his credit, Wes Brown, stuck to his task and refused to panic against the tricky Villa winger.
Gabby Agbonlahor showed his speed and tenacity but he too faded against United's resolute defence in which Nemanja Vidic and England stalwart, Rio Ferdinand, were stalwarts.
The hard-working Gareth Barry attempted to inject some fighting spirit in to Villa but at the end of the day United's goalkeeper, Edwin Van der Sar, did not have a single shot to save.
United often appeared to be in second gear. Ronaldo flitted in and out of the match, while Louis Saha could make no impact against Martin Laursen and Curtis Davies.
Anderson was often a driving force for United but, after his initial darting run, he had to settle for a midfield battle with Nigel Reo-Coker.
But United managed to finish with their customary flourish to give them an easy looking win.
The turnaround in fortunes did not materialise until the final ten minutes. Rooney put Giggs in possession with a low pass and the Welshman's cross was bundled home at the far post.
Ronaldo was involved in the second goal when his shot was blocked but Rooney was on hand to hammer his shot home past an uncertain Scott Carson.