The UK is linguistically rich but faces a reducing uptake of language learning in schools, as pupils feel more withdrawn and disinterested in learning an additional language to English. A key component in many linguistic minority communities to preserve their language is complementary schooling (CS), which has wide-ranging educational and societal implications. This study compared social developmental outcomes and patterns among bilingual children who attended CS and those who did not in London. A sample of 153 pupils aged 4-9 years was recruited including 73 across five CS settings and 80 across four state primary schools. Measures included strength of ethnic and national identities, cognitive, athletic and social competences, exposure and proficiency for each language, and family affluence (FA). Results showed that CS-attendees reported higher proficiency in HL, particularly in literacy, compared to non-attendees. Ethnic and British identities were positively associated with the respective language's proficiency and exposure in both groups, while unique patterns were also observed. Regression models confirmed the unique contributions of HL and CS to ethnic identity, but age and FA were also significant predictors of English proficiency and social competencies. Findings indicate the supportive roles of CS and HL and identity development. Further considerations of these and other factors in subsequent research are discussed.