Prince Harry dropped in to see his grandmother with wife Meghan, Duchess of Sussex, on the way to the Invictus Games in the Netherlands last week.
He said in a US television interview it allowed him to check the monarch, 95, has “the right people around her”.
“It was really nice to catch up with her,” he told NBC’s Today show.
The duke also used the interview in The Hague, in the Netherlands, to describe the United States as his home “for the time being” where he has been “welcomed with open arms”.
Describing the visit to see the Queen, which was not announced in advance, the duke said: “Being with her, it was great. It was so nice to see her… she’s on great form.”
“She’s always got such a great sense of humour with me and I’m just making sure she’s protected and got the right people around her,” he added in a preview of the full interview, expected to air in the US later.
Prince Harry and Meghan, who had not visited the UK together since they stepped back from royal duties in early 2020, also saw Prince Charles while they were at Windsor Castle, reports said.
The 37-year-old did not attend a memorial service for his grandfather, the Duke of Edinburgh, in London last month.
He is currently engaged in a legal case against the UK government over his police protection in the UK.
Meanwhile, the Queen – who is 96 on Thursday – has been forced to pull out of several key events – including a traditional Easter Sunday church service – in recent weeks after speaking about her mobility problems.
The short clip from US television doesn’t reveal what Prince Harry was “making sure” the Queen was protected from. Problems with her health? Bad advice?
But his comments, following his flying visit to the UK, are likely to raise some eyebrows and maybe even some hackles among those already looking after the Queen in Windsor Castle.
There’s a danger of overanalysing a few seconds of small talk – and maybe more will be explained in the full interview later.
But what did seem clear is that Prince Harry sees his future as being in the United States, at least for the time being. Never mind the royal genes, it’s jeans and sunshine for now.
With a young family now growing up in California, it has the feel of putting down roots.
Prince Harry is visiting Europe in his role as private patron to the Invictus Games, which sees 500 injured and sick military personnel and veterans from 20 nations compete in a week of adaptive sports.
The aim of the event, which he founded, is to help recovery, support rehabilitation and promote wider understanding of injured and sick servicemen and women.
A film crew is expected to be joining him at the games as part of filming for a Netflix series called Heart Of Invictus.
Prince Harry and Meghan made a deal with Netflix in September 2020 to make a range of programmes which they said would focus on creating content that “informs but also gives hope”.