"I waited a long time, an hour or two, to make that picture perfect," said Mario in an interview with The Telegraph. "But I wasn't totally satisfied. Then, when I'd finished the shoot, they were about to leave and they suddenly hugged in front of a radiator.
"It was spontaneous emotion" he added. "You could see they were completely in love."
Having photographed Diana, Princess of Wales, for her famous shoot for Vanity Fair, Mario compared the late Princess and her sons.
William "is more like his mother," according to the Peruvian photographer. "There's a kindness and a gentleness there that instantly wins you over."
Mario added that he felt the Duke was completely "in love with Kate" when the trio worked together.
Harry is more like his father Prince Charles, says Mario, who noted that the 29-year-old has a similar sense of humour to the Prince of Wales.
Mario Testino at William and Kate's wedding
Of the Duchess of Cambridge, Mario said that she "is beautiful in a different way" to Diana.
"She radiates happiness," he said. "She is comfortable in her love."
Mario, 59, who has recently been awarded an OBE for his services to photography and charity, first met William and Harry in 2008 when he was asked by Kensington Palace to showcase a series of his portraits of Diana and wanted to show the exhibition to the Princes first.
Mario, who was a hot contender to take the official christening pictures of Prince George before Jason Bell was chosen for the role, recalls that William and Harry brought their then girlfriends, Kate Middleton and Chelsey Davey, to see the photographs.