Justin said Mbuso overheard him asking the security guard to deliver the card to the former president and said to him: "Tata isn't too busy, he's reading the papers and I'm sure he wouldn't mind a visitor."
"It was really amazing to get to walk through their grand house; Madiba was sitting in the lounge. When he saw me, he told me to go closer, held my hand and said that my card was lovely," said Justin.
The grade 6 pupil at Johannesburg's German International School was not sure what to do for his 67 minutes of community service on the day so he made a card to thank Madiba for all he had done for South Africans and give clothes to charity.
After donating the clothes in Sandton, Justin asked his dad, Michael, to take him to Houghton so he could drop off the card.
On the card, Justin had written: "I wish you a happy 92nd birthday and all the best.
''It broke my heart when I heard about the death of your great-grandchild. I wanted to thank you for everything you did for South Africa."
Justin drew four cartoons: one depicting Mandela in prison in 1970, a poster of the Free Nelson Mandela campaign at Wembley Stadium in 1998, the Union Buildings in 1994 and Soccer City in 2010.
Mbuso took the picture of them with Justin's camera.
Justin said that when he told friends at school about his experience they did not believe him at first.
"Luckily I have pictures to prove it.
"When I think about it I feel like I was dreaming. I never imagined that it would be so easy to meet Madiba," he said.
His dad was not so lucky. He waited in the car while Justin went to drop off the card, and only learned of the meeting when his ecstatic son returned.