Purpose: This study aims to examine the associations of phonological, lexical, and grammatical skills within and between languages in Mandarin-English bilingual preschoolers. Method: Sixty-three Singaporean Mandarin-English bilingual children aged 3-5 years were assessed for articulation, receptive vocabulary, and receptive grammar using standardized instruments in English and compatible tools in Mandarin. Regression analyses were performed on each language outcome, with other language variables as predictors, controlling for age, nonverbal working memory, and home language environment. Results: Phonological and grammatical skills in one language predicted corresponding skills in the other. Phonemes shared across languages showed higher accuracy rates compared to unshared phonemes, while accuracy varied across grammatical structures. Vocabulary did not correlate between languages. It was influenced by household language distribution, with Mandarin vocabulary also correlating with nonverbal working memory. Mandarin grammar positively correlated with the number of native Mandarin speakers at home. Within each language, phonological skills were predicted by vocabulary, while vocabulary and grammar were reciprocally predictive. Cross-language, cross-domain relationships were weak. Conclusions: This study shows domain-specific cross-language associations and language-specific cross-domain associations in Mandarin-English bilingual children, indicating both interdependent and autonomous development. Our findings call for approaches that value the child's full linguistic repertoire and utilize interconnectedness between languages and language domains to enhance bi/multilingual competence. They also highlight the importance of assessing each of the child's languages and considering individual bilingual profiles in research on bilingual language development.