This study investigated the experience of and effect created by a team sport and coaching intervention upon the self-concept of male school students (age: 12–16; n = 129) from diverse backgrounds. A convergent parallel mixed-method design was used to compare and relate a questionnaire study and an interview study in a quasi-experimental design, based on a two-year intervention period. Quantitative results showed a significant effect on (1) general self-concept, (2) physical skills, and (3) social relations. Qualitative interviews showed participants experiencing (1) a stronger and more confident self, (2) a better physical condition, and (3) a stronger and more supportive social environment. Implications of the results are discussed alongside strategies for working with self-concept in school settings targeting migrant background boys.