Tolu Duckworth, an A-level student at Townley Grammar School in Townley Road, Bexleyheath took the title at a three-day contest.
The event saw the best of the first aid charity’s young volunteers from across the country compete against each other.
Tolu, 17, said: “Honestly, I feel ecstatic. It’s such an honour and it’s a very overwhelming feeling knowing that my hard work paid off.
“There was a lot of great competition so I’m still waiting for the shock to finally settle. It’s definitely weird too because I now have so much attention and I’ll need a while to get used to it all.”
The event in Nottingham took place over the weekend of February 9 to 11 and saw 31 young people vying for honours.
The competition tested the youths on first aid communication, teamwork and presentation skills.
Tolu said: “I was shaking with nerves on the train journey to Nottingham but that was definitely eased once I met the amazing people I now call my friends.
There was so much support provided so I was pretty much calm and collected for most of the competition.
“I was able to just be myself which can sometimes be hard and I guess this is what made the presentation and interview a lot easier to go through.”
Tolu joined St John Ambulance Erith Unit more than three years ago and she will now represent fellow cadets from London and the South of England.
Tope Duckworth, Tolu’s mother, said: “I am so proud of her hard-work, determination and efforts because she never stops trying and always pushes herself to be the best she can be.
“It was such a relief receiving that phone call after the ceremony as I was definitely more nervous about it all than she was.
“Now she has all these high-profile events she can go to and I can’t help but be a little jealous.”
St John Ambulance is a charity that provides transport and ambulance services across England.
Martin Houghton-Brown, St John Ambulance’s chief executive, said: “We had some brilliant candidates.
“I’m very excited to have a new cohort of talented cadets who will be fabulous ambassadors for our important work in the community.”
Cadets are aged between 10 and 17 and they are taught advanced first aid skills.
To find out more about St John Ambulance’s youth programmes, call 08700 10 49 50 or visit www.sja.org.uk/young-people