The charity, which supports islanders with learning disabilities, said the annual fundraiser typically raises between £10,000 and £15,000 and attracts hundreds of participants.
Hallie’s parents both work in care, so she understands the importance of Mencap’s work.
“I want to raise money and help the people you help,” she told her Mum and Dad.
Hallie’s grandfather Chris Gray, said: “Hallie-Rose was given a certificate and was informed that she is the youngest child ever to complete the [Jersey Mencap South Coast Charity Challenge] course unassisted.
“She is extremely pleased with herself and has already told them that she wants to sign up for next year’s walk.
“To say we are proud of her achievement would be the biggest understatement of the year.”
The Samarès student is also growing her hair to raise money for the Little Princess Trust and plans to donate her locks to charity.
“She has a heart of gold,” said mum Chloe Gray.
To donate to Hallie’s fundraiser, click HERE.
