This paper investigates shift in language dominance in sequential bilinguals following migration. Participants include 149 Polish-English bilinguals who relocated to the United Kingdom in early adulthood and underwent processes of acculturation and sociocultural integration. The independent variables in this study are divided into (1) sociocultural aspects, including: migration, acculturation level, social network profile, predicted future domicile; and (2) biographical aspects, including: age of onset of L2 acquisition (AoA), age at migration, and length of residence. The study employed both quantitative and qualitative approaches. The findings revealed strong links between language dominance and sociocultural variables including acculturation level, social network profile, and predicted future domicile. The results showed that sociocultural integration is a strong predictor of shift in language dominance. This study adds sociocultural and dominance perspectives to current research on language development over lifespan, by documenting a shift in language dominance in young adult sequential bilinguals following migration.