A career becomes sustainable when it enables one to remain happy, healthy, and productive. However, the lack of empirical research evaluating the effects of Work-Life Balance (WLB) on all three indicators of career sustainability in the same research setting is a significant knowledge gap in advancing research on sustainable careers. This research aimed to analyze the role of WLB in sustaining employees' careers, investigate the mediating role of competency development initiatives, and examine the moderating role of age in the Netherlands and Pakistan. The study analyzed data from 760 employees in the Netherlands and Pakistan using a quantitative survey-based research methodology. Convenience sampling techniques were used to collect data. Structural equation modeling was used to analyze the data. The results demonstrated that WLB is associated more positively with indicators of sustainable careers for Dutch employees than for Pakistani employees. The study also uncovered differences between the two samples' mediating mechanisms for competency development initiatives. With improved WLB, Pakistani employees become more future-focused and participate in career management activities, while WLB increases Dutch employee participation in learning activities and competency development. Age was determined to be a weak moderator, as the positive effect of WLB on competency development initiatives was only diminished in the Pakistani sample of older employees. This study contributes to the literature on sustainable careers by demonstrating that, while WLB is important for the career sustainability of employees in both cultures, in the individualistic culture of the Netherlands, the career sustainability of employees is more sensitive to their WLB experience. As a result, organizations must invest in providing WLB to their employees, as it is essential for their careers' sustainability.