Kate, 35, was speaking to Jamie Maloy, headteacher of Viking Primary School in west London, during a reception for London Marathon runners at Kensington Palace today.
Mr Maloy lavished praised on the prince, saying: 'What I love is your brother-in-law talking the other day, because that's just ignited it.
'Everyone's talking about mental health now because someone who is so famous and so much in the public eye has actually turned around and said something hit me hard, the death of a close loved one. And I think people have gone "good on you, because you could have never mentioned that."'
When he told Kate how Harry was 'a real hero to a lot of people', she replied: 'Absolutely. He's been brilliant.'
The Duchess was hosting a group of athletes running for Heads Together on Wednesday, the mental health campaign she launched last year alongside the Duke of Cambridge and Prince Harry which has been chosen as the official charity for this year's race.