The 48-year-old locked horns with the Scot ever since he took over at the Etihad Stadium, and while their relationship often became fractious the Italian always retained respect.
Mancini described the Manchester United boss as the best manager of his generation and revealed how he relished going up against the legendary Scot in their Manchester derby rivalry. He also described the soon-to-retire United boss as the greatest manager of his generation.
"For me, it has been a great honor and pleasure to compete against Sir Alex for three years," Mancini told reporters. "It was a great honor to beat him in Old Trafford and I wish him good luck for his future. I don't know for which reasons (he has retired).
"Everyone can have his opinion but one manager who stays for 27 years in the same club and won every trophy for 27 years - it is an incredible situation.
"I don't think there will be another manager like him.
"It is difficult to say Sir Alex is the best (ever), but he is the best in the last 27 years."
When asked whether Sir Alex's retirement will tip the title balance in City's favor, Mancini replied: "No. I think that when there is a top club like United, you can change the manager, an important manager like Sir Alex who built this team, but I think the club will stay a big club."Mancini recalled his two wins at Old Trafford as special moments in his managerial career and also claimed he sent a message to Ferguson following the 71-year-old's retirement.
"For me it was fantastic to compete against him in every derby because he was a top manager," Mancini said. "I think we should respect his decision."