The competition challenged young artists to come up with their own mythical creature to help transform ten acres of the Museum Gardens into a Wonderland using lighting, large scale projections and new technologies.
Organisers said they had 'a lot of entries' and The Mount's Eloise Hewson had her work, "Tribal Protector" chosen as the overall winner by comedian, artist and guest judge Vic Reeves.
Eloise said of her win that she was "Really surprised. We had been working on it in art group for two weeks. We were also doing a music competition and I was thinking about Hakuna Matata, so I had a lot of African ideas in my head, and the Tribal Protector is what came out."
Eloise's mother, Helen Hewson, said "I'm really pleased that the School entered Eloise for the competition. It's marvellous she's had that opportunity."
Jo Atkinson, Art teacher at The Mount School, said "We are all thrilled that four of our girls are in the final selection of winners in their respective categories.
Here at The Mount we encourage each girl to find her own creative voice and to think independently – to think outside the square.
The girls' work and inspiration is entirely their own, which shows in the unique individuality of each piece. I am so proud of Eloise, Ruby, Ailish and Harriet for having the courage to enter this competition for the Illuminating York Festival, and Eloise must be over the moon that her "Tribal Protector" is the chosen overall winner."
The 11-16 years category was all Mount hat trick, with Ailish Brown, Ruby Haswell and Eloise all chosen as the three winners. Harriet Hymas from The Mount Junior School was a winner in the 4-11 years category.
All the winners' creatures will be turned into projections which can be seen at the York Museum Gardens Hospitium during the Festival.
Counsellor Sonja Crisp, cabinet minister for Tourism, said "This competition was a fantastic way for people to get creative and involved in the artwork of Wonderland."
The Mount is an all girls boarding and day school in York with a strong reputation for excellence in the arts, maths and sciences and performing arts.
Earlier this year The Mount became the first school in England to introduce the international PeaceJam Foundation's Ambassdor programme, devised by Nobel Peace Laureates, into the curriculum.