But his excuse for missing the memorial has been called "pathetic" by Prince Philip's former protection officer of 14 years Richard Griffin.
"Prince Philip was their grandfather so William has obviously had a great training from him and is going to learn examples from him. I just hope Harry gets some of these things in his mind," he said.
"It [Harry not being here] was a big disappointment for everybody. People were talking about it. Certainly around where I was people were saying he should have been here.
"All this nonsense about how he couldn't get protection, as far as I'm concerned that was a pathetic excuse. He should've been here to honour his grandfather.
"If he was that worried about security, he could've stuck with his brother and father who have got wonderful security and he would have been more than safe."
Others pointed out that Harry was the only senior royal to miss the occasion, with Good Morning Britain presenter Richard Madeley asking, "What is he playing at?"
Royal writer Angela Levin tweeted, "Heart warming to see the great grandchildren at Prince Philip's thanksgiving service. Plenty of royals, and hundred of those connected to his charities. No one will forget or forgive he who didn't feel safe."
Meanwhile, royal fans took to Twitter to call out Prince Harry, with one writing, "I always defend Prince Harry no matter what but now he can royally kiss my fat a**. He's far too important to attend his granddad's memorial."
Another said, "It's shameful that Prince Harry refused to attend, but this is the same guy that trashed his family on global TV while his grandfather lay seriously ill in hospital."
A third added, "Sorry, but Prince Harry should be there, no question and no debate.
"He probably has 'better things to do' in the United States of America ... a disgrace of epic proportions."
Prince Harry last returned to the UK eight months ago, when he unveiled a statue of his late mother Diana in London last July with his brother Prince William.