The treatment was developed for Bardet-Biedl syndrome (BBS), a condition which leaves people blind by early adulthood.
Catherine L’Estrange, who was diagnosed when she was a baby, underwent a procedure at St Helier Hospital in March to receive the gene therapy developed by biotechnology company MeiraGTx.
Surgeons removed the jelly inside her eye and injected healthy copies of a gene into the retina – the light sensitive layer of tissue at the back of the eye.
The procedure has only been performed on one other person in the world.
BBS is caused by mutations to one of 20 different genes and impacts around one in 100,000 births in the UK.
For people with the condition, small cells in the retina die, leading to blindness.
But if the treatment works, Catherine’s sight will be stabilised or may even improve, according to the surgeon who treated her.
Catherine, who lives in North Acton, west London, hopes the procedure will allow her to continue one of her favourite hobbies: reading books.
“If this treatment works, it will help me to carry on seeing things around me, and most of all I will be able to carry on reading books, which is one of my favourite things to do,” she said.
Catherine’s father, Reverend Timothy L’Estrange, said most children with BBS do not get diagnosed until they are in primary school, but his daughter was diagnosed at a few weeks old, allowing the family to plan her future.
He said: “Our policy was to develop her independence and resilience as much as possible, ready for the inevitable loss of her sight, which began with her becoming night-blind, then colour-blind, and continued with her losing her peripheral vision.
“We were told that possible gene therapy was many, many years away, and was likely to arrive after Catherine had entirely lost her sight.
“So we were surprised and delighted when we learned this treatment had become available, and that Catherine would be one of the first patients in the world to receive it.”
The new treatment is designed specifically for patients with the BBS10 gene mutation, which is one of the most common.
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Only one other person – a 17-year-old girl from Canada – has received it before Catherine.
The parents of the girl, who wishes to remain anonymous, said: “This treatment has given our daughter a precious chance to preserve her vision, and we hope it will become a life-changing therapy for children with BBS10 all over the world.”
The procedure has also been performed on another younger child with BBS since Catherine.
Only one of Catherine’s eyes has been treated with the new gene therapy, with her team now waiting to see the results.
Neruban Kumaran, consultant eye surgeon at Epsom and St Helier University Hospitals NHS Trust, said patients had already reported better vision in dim light.
He added that the feedback so far has been positive, though it will take years before the full results are known.
He told Sky News: “Over time, many months and years, we’re looking to see if her vision has been protected, so she stops losing vision.
“We look to see if there’s protection from gradual loss of vision.
“Children with this condition often have poor vision in dim light, so we would look to see if they maintain their vision in dim light, or that improves following this treatment.”
He said the gene therapy could pave the way for more treatment options for inherited retinal diseases.
“It’s a step forward in a large body of work that offers a lot of hope for a lot of children,” he said.
Mat Shaw, chief executive of St George’s, Epsom and St Helier Hospitals Group, said: “As a father, I can’t imagine how it feels to watch your child slowly go blind.
“I’m so proud that our teams are offering hope to these children and their families.”
