The 21-year-old Portuguese winger scored twice, hit the bar and led Reading a merry dance until he was replaced by Darren Fletcher in the 79th minute.
The goals were Ronaldo's 11th and 12th in the League campaign and he looked set to smash through the 15-goal target set him by Ferguson at the start of the season.
Ronaldo has now scored two goals in each of his last three games, yet United were made to fight all the way by a Reading team which never gave up and put the leaders under pressure in the dying minutes.
The Royals, who held Chelsea to a 2-2 draw at Stamford Bridge on Boxing Day, had substitute Sam Sodje sent off by referee Mike Dean in the 68th minute.
The Reading defender had been on the pitch for only nine minutes after replacing Graeme Murty. But after being booked for a foul on Gabriel Heinze, he was dismissed four minutes later when he pulled back Wayne Rooney.
But the day belonged to Ronaldo. Denied in the sixth minute when he cut in from the right and saw his shot bundled away by Reading goalkeeper Marcus Hahnemann, Ronaldo went from strength to strength.
Hahnemann had a mixed game with brilliant saves interspersed with a failure to gather shots at the first attempt.
United took the lead in the 33rd minute with a goal in which Ronaldo played his part. Found by Heinze to the left of the Reading box, he beat Glen Little before crossing to the near post where Ole Gunnar Solskjaer stole in before Ibrahima Sonko to head home from eight yards.
United's lead lasted five minutes. Nicky Shorey floated his free-kick from the right into the centre of the United box and Sonko got in before goalkeeper Edwin Van der Sar and Heinze to send his back header over the Dutchman and into the net.
United were unlucky not to restore their lead within a minute. Michael Carrick drove his corner into the centre of the Reading penalty area and Ronaldo beat Sonko to send his header crashing against the bar.
Ryan Giggs replaced Ji-Sung Park at the start of the second half and his arrival was to prove crucial to United.
Rooney twice went close before United made it 2-1 in the 59th minute. Giggs found Rooney on the left corner of the penalty area and his cheeky flick sent Solskjaer away to his left.
The Norwegian beat Sonko and curled his shot beyond Hahnemann. The ball hit the far post, but Ronaldo followed up to drive in the rebound from eight yards.
Giggs played an even greater part in United's third goal after 77 minutes. His cross from the right eluded the Reading defenders and Ronaldo was standing unmarked four yards out on the far post to hook the ball home.
But United were to suffer a nervous finale. With the game seconds into injury time, Hahnemann's long kick was headed on by Ivan Ingimarsson to Leroy Lita on the right.
Lita cut into the United box and beat Mikael Silvestre before rolling his shot past the oncoming Van der Sar and into the net from eight yards.