The Peruvian photographer shot to worldwide fame when he snapped the late Princess Diana for Vanity Fair in 1997 and when he was asked to take the official engagement portrait of her son and his fiancee, it felt as though he was giving something back to a country that had given him so much.
He told Grazia magazine: "I never met the princes until they came to see the exhibition of the pictures of their mother. Doing the official engagement portrait of Prince William and Catherine Middleton felt for me like I was paying them back.
"This country gave me a job, educated me, tolerated me to exist. I am grateful for that."
After shooting William and Catherine - who married in April - Mario could see the young couple were absolutely besotted with each other and their love filled his heart with "joy".
He said: "I am very happy to have been asked to cover this historic moment that the whole world was waiting for. They are in their prime and brimming with happiness. I have never felt so much joy as when I see them together."