This paper discusses how linguistic diversity is deployed and negotiated by the Taiwanese Japanese writer On Yūjū in postcolonial and transnational context. Parallel to her personal life as a migrant writer, On’s protagonists experience considerable tension between languages, ethnic identity, national allegiance, and cultural differences due to their mixed Chinese, Taiwanese, and Japanese background. These continuing conflicts are sharpened in her Japanophone writing greatly inflected by the other two linguistic traditions. Incorporating her mother tongue (Taiwanese Hokkien), “motherland tongue” (Mandarin), and “stepmother tongue” (Japanese), her works unfold stories about divided identity and migrant experiences in the multilingual “mama tongue” (a mixture of all three languages) as her mother speaks in everyday life. On’s analogy between matrilineal kinship and her languages reveals both the connections and challenges in acquiring, practicing, and owning these three languages across national boundaries and cultural membership. Instead of the prevailing nativist icon of Taiwan as an “orphan of Asia” devastated by serial colonization of Japanese empire, Chinese settler regime, and American dominance, On advances the trope of “child in the middle” who navigates different political powers, languages, and cultures without subjecting herself exclusively to any. With a detailed examination of linguistic strategies in On’s works, this paper analyzes how ownership of languages is legitimized and deconstructed in tandem with transnational movement, neoliberal citizenship, and mobile belonging in postcolonial East Asia.