Bruno Fernandes added a second as United broke from deep inside their own half in the final minute of injury-time.
After a tepid opening period, Ronaldo struck six minutes into the second half.
The Portuguese collected Scott McTominay’s pass before edging right of the Brighton defence and, with barely any backlift, sent a low shot curling beyond a static Robert Sanchez.
Three minutes later, Brighton skipper Lewis Dunk was sent off after VAR sent referee Peter Bankes to review the defender’s foul on Anthony Elanga.
Fernandes wasted a glorious chance when he failed to get past Sanchez with only the Brighton keeper to beat, before Ronaldo nodded an excellent chance wide of the far post.
It meant United breathed a sigh of relief when Jakub Moder’s speculative effort from the corner of the box bounced back off the bar with David de Gea beaten.
Brighton were still trying to salvage something from the contest when Fernandes struck with virtually the final kick.
Jekyll and Hyde United
In his pre-match interviews, United’s interim manager Ralf Rangnick spoke about his team needing to be more Dr Jekyll and less Mr Hyde. Sadly for the German, the first half had far more evidence of the latter, evil, side.
When United were pressed with any great intensity, they gave the ball away. Restored to the starting line-up ahead of Paul Pogba, Fred was one culprit when he delivered a straight pass intended for Ronaldo straight to Adam Webster, who was not much more than a couple of yards away.
But the Brazilian was not on his own. Indeed, as Ronaldo drove a free-kick in a promising situation at the end of that disappointing opening period into the wall, it was hard to think of a redeeming feature for the hosts.
They did create one decent chance, when Ronaldo’s backheel sent Jadon Sancho running into the box. But once Brighton keeper Sanchez had turned the England man’s shot away, the best bit of United play was De Gea’s excellent save to deny Moder’s header.
The boos that accompanied the half-time whistle were predictable and when Pogba emerged late after the break and missed the start of his warm-up, it appeared United’s shambolic evening would continue.
Instead, the Frenchman was on the touchline to join the applause as Ronaldo struck the crucial 51st-minute opener.
Ronaldo knew he should have had more goals, though, and kicked an advertising hoarding in frustration as a header from a corner went narrowly wide.
However, after an unsatisfactory period for both player and team as leads have been lost in three successive games, at least this time they were able to secure a much-needed victory.
Familiar losing feeling in Manchester for Brighton
Given they have now drawn two and lost 12 in 14 away games at Manchester United – and not won in 20 trips to face either City or United – the final outcome cannot have been a surprise to Brighton.
Yet, despite the scoreline and losing centre-back Dunk, there had to be an element of this being a missed opportunity for the visitors.
For long periods in the first half, they were by far the better side but, not for the first time, the lack of a potent finisher cost Graham Potter’s men.
Moder could not have done much more with the header that De Gea turned away, nor the shot that hit the bar.
But Yves Bissouma fired wide before the break when placed to do much better. Danny Welbeck then came agonisingly close to snatching a draw against his former club when he sent his header over from eight yards after being picked out by fellow substitute Tariq Lamptey in the final minute.