United have been linked with an audacious move to take the Portugal international back to the Premier League five years after he moved to Madrid.
However, Mendes says it would be "impossible" to lure the 29-year-old away, insisting he is very happy in Madrid.
The agent expects Ronaldo, who has scored 13 goals in six La Liga matches this season, to extend his contract when his current deal runs out in 2018, suggesting he will stay at the Santiago Bernabeu until he is 40.
"Cristiano Ronaldo will retire at Real Madrid when he turns 40," Mendes told Cadena Cope. "He is very happy at Madrid. It is impossible to lure Ronaldo away from Madrid. He breaks all the records and is regarded as the best athlete ever.
"He is happy at Real Madrid and has always been happy here. He will break all the records and then retire at Madrid.
"Manchester United will always have a place in his heart, because he spent six years there. But he is only thinking about Madrid now and will always stay here.
"He is delighted to be at Madrid. People know that it is impossible to sign him, so it's not even worth calling."
Mendes also believes the FIFA Ballon d'Or winner is already the greatest footballer in history.
"Ronaldo is the best player in the world and the best we have ever seen," he added. "You will never see a player like him again.
"He is getting better and better and will continue to do so until he turns 40.
"You cannot compare him with anyone. There will not be anyone like him in the next 500 years."
