A successful school-to-work transition is an important yet challenging step in graduates' careers. Unfortunately, most interventions found to help students master this transition are too elaborate and time consuming to scale-up to student cohorts, as they require multiple sessions and/or one-on-one counseling. In this study, we answer to the call for more research on theory-based yet efficient career interventions that can help large numbers of students prepare for this transition. Based on career construction theory, we present and validate a scalable career intervention combining online assessments with one or two short workshops to foster students' career adaptive responses and – through this – their career adaptability, as well as their subsequent quality of employment. To evaluate the optimal intensity of the intervention, a quasi-field experiment addressed the development of career adaptive responses and adaptability between three intervention groups (n = 48, n = 302, n = 42) and a control group (n = 79) over three time points (pre-intervention, post-intervention and six months later). Structural equation modelling showed significant indirect effects from partaking in the intervention on participants' perceived fit, career growth and satisfaction in their jobs through enhanced career adaptability. Effects regarding intervention intensity were somewhat less clear. In sum, results show that a theory-based compact, scalable and partly web-based career intervention may help students prepare for the school-to-work transition and raise their chances of finding high quality employment.