This article reports on a case study where a professional musician was assigned to a primary school as mentor in a project where 14 primary-aged children, with their teacher's direction, were involved in the composition of a piece of music that would act as prologue to the school's major production. The researchers were interested in aspects of the composition project that appeared to develop student's compositional skills and motivation and how the mentor might exercise his role in the composition project alongside the teacher, who was the school's music specialist. A range of data was analyzed: classroom observations, emails, questionnaires, and one-on-one and focus-group interviews. Findings suggested that the involvement of the mentor was well received by the pupils and that the mentor and teacher complemented one another in their enactment of both pedagogical and compositional roles. Students enjoyed a high sense of success in the production of their composition. Student learning were highly variable and a range of activities and processes was identified by students as contributing to their learning. (Contains 4 tables.)