This paper investigates a group of mainland Chinese students’ multilingual learning experiences in an English-medium university in multilingual Hong Kong. Informed by the sociological construct of investment, the study focuses on the role of identity and language ideology and their interaction in shaping the participants’ experiences of learning English and Cantonese and their multilingual development. The findings reveal that the participants’ multilingual investments were mediated by their ideologies of sociolinguistic competence and flexible multilingualism, which contributed to the development of their identities as competent multilingual speakers. However, the participants’ negotiations of their multilingual identities were constrained by the local students’ deficit perspectives on the participants’ multilingual competences as a result of the influence of the ideology of native-speakerism in the local society. The findings also show that the participants’ internalization of the ideology of neoliberal multilingualism and the ideology of multilingualism as indexical of cosmopolitan membership prompted their multilingual investments, which expanded their imagined identity options for the future. Taken together, the findings point to the complex and dynamic interaction between identity and language ideology in shaping multilingual investments. The study also expands our understanding of multilingual learning by contributing to the conceptualization of ‘multilingual investment’ from a sociological perspective.