The Spanish giants will face shock finalists Kashima Antlers, the J-League champions, in Yokohama on Sunday as they look to capture their second world title in three years and fifth in total.
"Our sole aim was to reach the final -- so it's job done," Real coach Zinedine Zidane told reporters after goals from Karim Benzema and Ronaldo extended his side's club record unbeaten run to 36 matches in all competitions.
"We didn't have a great rhythm in the first half, but in the second half we created more chances and Cristiano was able to kill the game off so I'm very happy," added the Frenchman.
"It was a long flight to Japan and we have very few days to rest and prepare for the final, but we have come here to win this Club World Cup."
His countryman Benzema said: "It was a hard game against a tough team. The conditions were difficult too with the long journey we've had, but we played well and won the game -- now for the final."
With Ronaldo firing blanks until scoring with virtually the last kick of the game, Benzema struck in first-half stoppage time, finishing clinically with the outside of his right boot after a deft pass from Toni Kroos.
Ronaldo, who scooped his fourth Ballon d'Or earlier this week, had already squandered a gilt-edged opportunity by heading against the post.
The Portuguese superstar also saw two stinging efforts blocked by goalkeeper Moises Munoz but his dogged persistence paid off when he swept home from a tight angle deep into injury time.
Confusion followed, however, when it appeared the goal could be ruled out using the video technology being trialled by FIFA in Japan.
Ronaldo's face turned to thunder as the Mexicans protested he had been offside -- until Paraguayan referee Enrique Caceres finally indicated that the goal stood.
"It's a new invention but to be honest I don't like it," said man of the match Luka Modric when asked about video replays.
"It causes confusion. I hope they don't continue with this system because for me it is not football."
Zidane refused to criticise the technology, although his comments hardly amounted to a ringing endorsement either.
"If you ask my personal impression, it could cause problems," he said. "But if it's FIFA policy then we have to get used to it. We don't have the power to decide. Maybe it will help make things clear and transparent."
The 31-year-old Ronaldo has hit a remarkable 377 goals in 366 games for Real with 118 more in 292 games for Manchester United and five in 31 for Sporting before that.
Club America coach Ricardo La Volpe insisted his side had been unfortunate to lose.
"I think the luck was against," growled the firebrand Argentine.
"Real Madrid were not the better team. I told the players they can be proud of the way they fought," he added. "It was a very tight game which we dominated for periods, but we were not aggressive enough. We just couldn't find the final ball."