This dissertation explores the career sustainability of forty-two European Union (EU) citizens living and working in another EU country. It examines how sustainable careers can be pursued, achieved, and maintained in a transnational context over time. The countries of focus are Ireland, Germany, and Spain, considered both as home and host countries for the study participants. Adopting a social constructionist philosophical paradigm, this predominantly inductive study analyses the participants' intra-EU mobility experiences, viewing them both from their own migration realities and from the broader perspective of intra-EU geographical labour mobility as a phenomenon. The research employs a whole-life perspective (Litano & Major, 2016; Hirschi et al., 2020) to analyse the transnational career sustainability of intra-EU migrants. The analysis integrates literature from international human resource management (IHRM), expatriation, and career studies, alongside interdisciplinary literature on migration and transnationalism. This approach aims to provide a deeper understanding of how participants construct and interpret their career experiences within a dynamic transnational context. The findings contribute to existing research on international careers and the sustainable career construct from a transnational perspective. Transnationalism is defined as "the processes by which immigrants build social fields that link together their country of origin and their country of settlement" (Glick-Schiller et al., 1992, p.1), operationalised here as transnational living, where participants maintain "sustained and similarly significant attachments, interactions and presences in two or more societies separated by national borders" (Carling, Erdal & Talleraas, 2021, p.2). In this dissertation, the constituent components of transnational living—social integration, social organisation, complementarity, rhythm, and balance—are detailed in Chapter Four (section 4.4, pp. 81-85), empirically explored in Chapter Six (section 6.3, pp. 124-145), and further discussed in Chapter Eight. Theoretically, this work establishes the case for applying the conceptualisation of transnational living—originating from the migration field—within the international business domain. The discussion chapter integrates the five dimensions of transnational living with the levers of a sustainable career to present a structuring mechanism of a transnational sustainable career (section 8.2.3, p. 208-214), which has been empirically validated in this PhD research undertaking. By integrating the concepts of transnational living and sustainable careers, this dissertation presents an organising framework for studying the sustainability of transnational careers, offering a novel perspective on dynamic inter-country career movements. The study reveals that the three indicators of a sustainable career—happiness, health, and productivity—intersect with and are influenced by the complementarity dimension of transnational living. These complementarities, which can be professional, relational, or related to general well-being in the host country, vary across different career and life stages, highlighting the temporal nature of both sustainable careers and transnational living. Empirically, this PhD dissertation provides a snapshot of intra-EU migrants' career experiences and their continuous associations and engagement with their home and host countries over time and space. In a world characterised by globalisation and mobility, this study describes how and discusses why internationally mobile individuals maintain ties with their home countries while residing in a host country, normalising the experience of living with a "foot in both camps." The study contributes to the international career and mobility literature by introducing the concept of "transnational living," which holds potential value for management and international HRM scholars. While this study looked specifically at the geographical setting of the EU, there may be other examples of particular settings facilitating transnational living (e.g. Australia to New Zealand). The main contribution of this study is the provision of an overarching framework of contemporary transnational sustainable careers which can be used to investigate the rise of transnational living as a phenomenon, beyond the EU, and transnational living as a contemporary means of adjustment.