As well as attending the event of the charity which is so close to his heart, Harry's beaming smile was no doubt in part due to him having seen Prince William and Kate Middleton's infant son at Kensington Palace the day before.
When asked about the latest addition to the House of Windsor, Harry said he would "make sure he has a good upbringing, and keep him out of harm's way and to make sure he has fun".
"The rest I'll leave to the parents," he joked.
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Harry said that he had already cuddled his tiny nephew George Alexander Louis who was "crying his eyes out" when he met him.
Uncle Harry visited the central London exhibition which features touching images taken by royal photographer Chris Jackson. The photographs that capture the hard work that goes on at Sentebale to improve the lives of the vulnerable children of the African nation Lesotho.
A variety of images are featured in the exhibition - from portraits of the unforgiving reality of daily life for orphans to the encouraging outcomes of those children with disabilities benefitting immensely from an education.
Cathy Ferrier, chief executive of Sentebale, said: "We have ambitious plans to expand, including building a specialist centre for children living with HIV which will allow us to reach many more young people in Lesotho.
"We would like to invite everyone along to the exhibition to see for themselves the difference we are making to the children."
Prince Harry founded the charity with Prince Seeiso of Lesotho in 2007 after visiting the country on his gap year and being compelled to help vulnerable children who are often victims of poverty and the HIV/AIDS epidemic.
They came up with the name Sentebale, which means "forget-me-not" in Sesotho, the language of Lesotho.
His involvement in the children's charity echoes the extensive work of his mother Diana, Princess of Wales.
"This charity is a way in which Prince Seeiso and I can remember our mothers, who both worked with vulnerable children and people affected by AIDS," Harry wrote on the charity's website. "I really feel that by doing this I can follow in my mother's footsteps and keep her legacy alive."
Harry most recently visited Lesotho in February 2013.
The free exhibition at the Getty Images Gallery is open to the public from 26 July to 10 August. Prints are available of all the photos and a percentage of proceeds goes to Sentebale.