Semantic integration is a mechanism of lexical-semantic processing. When indexed by N400, it emerges coupled with vocabulary spurt in the second year. To what extent maturation and language exposure contribute to its development remains unclear. Bilingual toddlers split their time between two languages. While experiencing similar concepts as monolinguals, bilinguals are less exposed to each language’s words. This makes them a key group to disentangle the relative contribution of maturation and language exposure in the emergence of SI. We investigate (1) whether bilinguals follow the same developmental trajectory of semantic integration as monolinguals, and (2) whether semantic integration differs between bilinguals’ dominant and non-dominant languages across time. If language exposure drives SI, we expect earlier timing of onset of semantic integration in monolinguals than bilinguals and language-dominance effects within bilinguals. In this event-related potential mixed longitudinal and cross-sectional study, bilingual and monolingual 18- and 24-month-olds were shown a picture-word priming-paradigm. We found N400 effect at 18 and 24 months in monolinguals. Bilinguals showed no evidence of N400 effect in either the dominant or non-dominant language at any time-point. Although the bilingual sample was smaller than planned, our results contribute to general neurodevelopmental and dual language acquisition models.