Previous literature has discussed the relationships between non-participation and underemployment in the labour market with health and wellbeing, including the impact of these circumstances on mental health. While it is sometimes argued that a mismatch between the actual number of hours worked and people's preferences about the amount of work they want has an association with mental health, findings are mixed. Moreover, job characteristics and age-specific factors are often not included in studies exploring these relationships. This paper examines the link between working hours mismatch, job strain and mental health in Australia, focusing on mature age workers (aged 45–64 years), a group of workers for whom these issues have seldom been explored. Results from our panel data estimations, using the Household, Income and Labour Dynamics in Australia (HILDA) Survey, show significant associations between a mismatch of working hours – that is, either working more or less than one's desired hours – and poorer mental health. Our analysis also suggests that jobs characterised by low levels of control and high demands have a significant negative impact on reported mental health scores. Designing flexible working environments and giving workers more autonomy should be priorities in order to improve the general mental health of mature age workers.