The 90-year-old duke of Edinburgh made headlines late last year when he underwent heart surgery, but he has since recovered and is again making personal appearances alongside his wife.
"Regardless of whether my grandfather seems to be doing his own thing, sort of wandering off like a fish down the river, the fact that he's there -- personally, I don't think that she could do it without him, especially when they're both at this age," Harry told Radio Times in a rare interview.
The prince went on to say the queen continues to attend functions even when she doesn't feel like it because she feels it is her duty.
"These are the things that, at her age, she shouldn't be doing, yet she's carrying on and doing them," he said.
The Mirror said Harry's comments will be included in the three-part documentary series "The Diamond Queen," which is to air on the BBC next month.
Prince William, duke of Cambridge, also is quoted in the program as saying his grandmother has "carved" out her own path between "private and public and duty" because "she's not had a blueprint," the Mirror said.
"I think she doesn't care for celebrity ... and she really minds about having privacy in general," William said. "And I think it's very important to be able to retreat inside and be able to collect one's thoughts and collect your ideas ... and then to move forwards."