Another Me tells the story of a teenage girl who believes she is being stalked by her double. It stars Tudors lead Jonathan Rhys Meyers, Welsh actor Rhys Ifans, veteran film star Geraldine Chaplin, and Sophie Turner of the cult TV programme Game of Thrones.
MacPhail, a multi-award author, writes children's and teen fiction that deals with gritty and thriller-orientated topics.
Described as a "dark, psychological chiller", About Me will have its first screening in Rome on November 15, followed by cinema releases in the United Kingdom, Spain, the USA and other countries.
The film has been produced by Rebekah Gilbertson and Nicole Carmen Davis of Rainy Day Films in London, in association with Fox International Productions.
Written and directed by award-winning director Isabel Coixet, the film, shot in Wales, also boasts Ms MacPhail as its executive producer.
She said: "When Isabel and the team at Rainy Day Films first contacted me about buying the film rights and explained their vision of developing the story for a teenage audience, I was really flattered. To see my story and characters come to life in a film has been an incredible experience.
"I was also really honoured to be asked to be executive producer. Working on the film was fun and a great privilege."
Ms MacPhail said she grew up dreaming of being a writer but it wasn't until years later, when she penned a story to help her daughter cope with bullying, that she began to write seriously. Her first full-length novel, Run Zan Run, went on to win the 1994 Kathleen Fidler Award for debut children's novels.
She has written a total of 40 novels, establishing a reputation as a writer of gritty urban stories for teenagers and children that tackle emotional and contemporary issues.
Some of the subjects she has covered to date include loan sharks, teenage pregnancy, gangs, and asylum seekers.