The rugby coach and motor neurone disease (MND) campaigner was one of 60 people to receive honours from William at a recent investiture ceremony at Windsor Castle.
Sir Kevin, who has raised millions for charity, received a knighthood for his services to the MND community, as well as for services to league and union rugby.
Speaking to BBC Breakfast, the prince said: “I think, frankly, he should have been Sir Kevin years ago.
“I don’t know how he keeps going, I’m worried about his knees, I’m worried about his hips. The guy is just a machine.
“He’s a top bloke, really nice guy, good, honest, kind, decent northerner and I love that about Kevin, that there is no ego with him. He is what you get.”
The Prince and Princess of Wales recently surprised Sir Kevin in Wakefield, West Yorkshire, as he completed the third day of his final 7 in 7 running challenges.
The challenge involved completing an ultra-marathon every day for seven days.
The fundraiser was inspired by his former Leeds Rhinos teammate Rob Burrow, who died in 2024 after living with MND for four-and-a-half years.
The prince said he had also written a letter to Mr Burrow’s family.
William (left) met Rob Burrow (right) and Kevin Sinfield during a visit to Headingley Stadium in Leeds in 2024 (Phil Noble/PA)
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“Everyone grieves differently and every circumstance is different, so you need to take your time over it,” he said.
“It’s always better to say something around grief than it is to ignore it.”
Speaking about receiving his knighthood, Sir Kevin previously said he was “massively humbled by the whole experience” and “very proud to accept it on behalf of the MND community and their families”.
The 46-year-old said William, who presented Sir Kevin and Mr Burrow with CBEs at Headingley Stadium in 2024, had been “fantastically supportive” of their campaign.
Sir Kevin told the Press Association: “I think he champions pulling communities together and I have seen how the MND community have come together on the back of Rob’s diagnosis and several other high-profile sportspeople over the last six or seven years.
“So, his support and, obviously, Princess Catherine’s as well, they’ve been wonderful and bring a real credibility to what we’re trying to do.”