The royal spoke during his and Kate Middleton’s first solo outing as the new Prince and Princess of Wales on Thursday.
The couple visited Sandringham, where they spoke with crowds and viewed cards and flowers left at Norwich Gates.
William added that walking behind his grandmother’s coffin during the procession from Buckingham Palace to Westminster Hall was “very difficult.”
“Don’t cry now, you’ll start me,” he told a woman who was present, according to a reporter who attended the engagement. Speaking of his and the Princess of Wales’ children, he said that while Prince George understood the loss of his great-grandmother, it was harder for Princess Charlotte and Prince Louis to comprehend.
Both William and Prince Harry walked behind their mother’s coffin when William was just 15 and Harry was 12.
The young boys were joined by their grandfather, Prince Philip, Diana’s brother, Charles Spencer, and their father, Prince Charles.
In a 2017 documentary, William told the BBC about the “very long, lonely walk” behind Diana’s coffin, calling it “one of the hardest things I’ve ever done.”
The prince said he tried to use his hair to hide, because he “felt if I looked at the floor and my hair came down over my face, no one could see me.”
He said it wasn’t “an easy decision” to join the procession, and described it as a “sort of collective family decision to do that.”
“There is that balance between duty and family, and that’s what we had to do,” William added. But there was a difference “between me being Prince William and having to do my bit, versus the private William who just wanted to go into a room and cry, who’d lost his mother.”
Prince Harry has previously spoken out about the walk, telling Newsweek in 2017, “My mother had just died, and I had to walk a long way behind her coffin, surrounded by thousands of people watching me while millions more did on television.”
“I don’t think any child should be asked to do that, under any circumstances,” the prince said, as the interviewer said his faced hardened recalling the memory. “I don’t think it would happen today.”
In an interview with the BBC two months after the Newsweek comments, the prince later said he didn’t “have an opinion whether that was right or wrong” to join the walk, and said he was “very glad” to have been a part of the day.
Harry and William reunited on Wednesday to walk behind their grandmother’s coffin. The two walked side by side, which was a departure from the funeral of their grandfather Prince Philip last year.
The two were separated by a cousin, Peter Phillips, during the Duke of Edinburgh’s funeral, which was pared down due to coronavirus restrictions.
Queen Elizabeth died “peacefully” on Thursday at Balmoral in Scotland at the age of 96. Her state funeral will take place Monday at Westminster Abbey. Over 2,000 guests are expected to attend.
After the service at Westminster Abbey, the queen’s coffin will travel to Windsor Castle for its final resting place at King George VI Memorial Chapel. She will be buried alongside her late husband, Philip, whose coffin will move to join hers.