This longitudinal investigation assesses the extent to which the gender composition of an occupation (e.g., the extent to which an occupation is comprised of males versus females) has an impact on mental health. We used 14 annual waves of the Household Income Labour Dynamics in Australia (HILDA) study to construct a measure representing the gender ratio of an occupation. The outcome measure was the Mental Health Inventory (MHI-5). A Mundlak model was used to compare within and between person effects, after controlling for possible confounders. Results suggest that males and females employed in occupations where their own gender was dominant had better mental health than those in gender-neutral occupations (between person effects). However, within-person results suggested that a movement from a gender-neutral to a male or female dominated occupation was associated with both a decline (females) and improvement (males) in mental health. These results highlight the need for more research on gender specific selection into and out of different occupations in order to progress understandings of gender as a social determinant of health in the work context.