She was born eight weeks prematurely and has cerebral palsy.
That meant her limbs, especially her legs, were stiff and rigid. It made walking, sitting and even sleeping difficult.
"Day to day living was a struggle for her," said Sophie's mother, Jane Donaldson.
But earlier this month Sophie had pioneering surgery at Alder Hey Children's Hospital in Liverpool.
The procedure is called Selective Dorsal Rhizotomy, and involves cutting nerve roots in Sophie's spine to relieve the stiffness in her muscles. Alder Hey is one of only five hospitals in the country where the operation is carried out.
Speaking before the operation, Sophie's father, John, said: "It's to give her a bit more confidence in herself, a wee bit more freedom."
During the surgery Sophie's body was wired up to numerous monitors. This allowed the doctors to see how her body reacts when they stimulate the nerve roots in her spine.
Neurosurgeon Benedetta Pettorini can then decide which nerve roots to cut, and which to leave alone.
"After three or six months the parents come to me and say 'It's a different child', and they can't even say why, it's just better," she said.
The first thing Sophie said to her parents when she came round after the operation was: "Mummy, Daddy, look at my legs - they're not tight."
Her mother told the BBC: "That was an early Christmas present. It was fantastic to hear that."
It is a long and complicated operation, but it is not a miracle cure.
Sophie will always need a frame to help her walk, but it should improve her quality of life and make walking and other movement much easier.
A fortnight after the surgery, physiotherapist Chris Sneade said: "Her movement is lot freer, she's more confident and her balance has improved. She's done fantastically."
Sophie now faces two years of physiotherapy and hydrotherapy to strengthen her legs. She has to use muscles she has not used before.
After three weeks at Alder Hey she and her mother are looking forward to being back home in Ballycarry for Christmas with Sophie's father and her eight-month-old sister, Emily.
Her mother said the staff at Alder Hey are fantastic, but being in hospital that long is "very physically tiring, so just to get home and relax and enjoy Christmas is going to be phenomenal".
Sophie Mathers is a cheerful and playful seven-year-old from Ballycarry near Carrickfergus, County Antrim.