Recent debates about whether the standard full-time working week (35–40 h) can be replaced by a shorter working week have received extensive attention. Using 2015 European Working Conditions Survey data, this study contributes to these debates by exploring the relationships between job quantity, job quality and employees’ mental health. Overall, we find that a job’s quality matters more than its quantity as measured in hours per week. The results show that actual working hours are hardly related to employees’ mental health but job quality, especially intrinsically meaningful work, less intensified work and having a favourable social environment, has positive effects on employee mental health, even in jobs with short working hours. Moreover, although working less than one prefers (under-employment) has negative effects, these negative effects become much smaller in size and non-significant in good quality jobs, especially in jobs with skill discretion and good job prospects. These findings develop the debates about a shorter standard working week by emphasising the continued and crucial importance of job quality in debates on the future of work. These results also suggest that policymakers should pay particular attention to job quality when addressing the dramatic reduction in total hours of employment in Europe following the COVID-19 crisis.