This study examined the development of vowel categories in young Mandarin -English bilingual children. The participants included 35 children aged between 3 and 4 years old (15 Mandarin-English bilinguals, six English monolinguals, and 14 Mandarin monolinguals). The bilingual children were divided into two groups: one group had a shorter duration (<1 year) of intensive immersion in English (Bi-low group) and one group had a longer duration (>1 year) of intensive immersion in English (Bi-high group). The participants were recorded producing one list of Mandarin words containing the vowels /a, i, u, y, ɤ/ and/or one list of English words containing the vowels /i, ɪ, e, ɛ, æ, u, ʊ, o, ɑ, ʌ/. Formant frequency values were extracted at five equidistant time locations (the 20–35–50–65–80% point) over the course of vowel duration. Cross-language and within-language comparisons were conducted on the midpoint formant values and formant trajectories. The results showed that children in the Bi-low group produced their English vowels into clusters and showed positional deviations from the monolingual targets. However, they maintained the phonetic features of their native vowel sounds well and mainly used an assimilatory process to organize the vowel systems. Children in the Bi-high group separated their English vowels well. They used both assimilatory and dissimilatory processes to construct and refine the two vowel systems. These bilingual children approximated monolingual English children to a better extent than the children in the Bi-low group. However, when compared to the monolingual peers, they demonstrated observable deviations in both L1 and L2.