PurposeThe initial transition into work has become less predictable. Therefore, emerging adults should take charge and be proactively engaged in their careers during the preparation stage of the school-to-work transition (STWT). We explored which behaviors emerging adults display during the STWT to foster their happiness, health, and productivity, how various contextual factors enable or hinder these behaviors, and to what extent these behaviors can be considered proactive.Design/methodology/approachTaking a qualitative approach, we conducted 44 semi-structured interviews with undergraduate students at an Applied Sciences University in the Netherlands six months before graduation. Additionally, we held nine focus groups (n = 55) and four interviews (n = 6) with contextual stakeholders (i.e. parents, faculty/staff, employers, the board of the university, higher education policymakers, and the Ministry of Education).FindingsThe students mentioned three main behaviors to foster their health, happiness, and productivity, namely, eating healthy food, maintaining social contacts, and reflecting on their motivations. Our analysis shows that, conceptually, none of these behaviors can be considered truly proactive. Moreover, the findings revealed multiple systemic underlying contextual hindrances to portray these behaviors, such as educational system characteristics, which make proactive behaviors less likely.Originality/valueAs the STWT is affected by multiple contexts, adopting a multi-stakeholder perspective is imperative when studying the phenomenon. We adopted the sustainable careers framework as a lens to uncover how emerging adults may build early career sustainability—additionally, we nuance current research on proactivity by concretizing the conceptualization of proactive behaviors.