Although males constitute a substantial proportion of carers until recently there has been little focus on the impact of caring on their mental well-being. This study aimed to examine the extent to which a variety of contextual, caring related and protective factors predicted the mental well-being of middle-aged male carers informed by Pearlin’s Stress Process Model. The study used secondary data from the Understanding Society study. Cross-sectional analysis of how factors relating to the role of caring, interferences of caring on employment and social participation and leisure predict mental well-being measured by the GHQ-12 was undertaken. Longitudinal analysis following those that became carers and potential changes in social participation, satisfaction with leisure and mental well-being was also undertaken. Data from 8,063 middle-aged men (1,612 carers and 6,451 non-carers) was used for the cross-sectional element and data from 4,665 (614 carers and 4,051 non-carers) was used for the longitudinal element. An estimated 20% of middle-aged men were carers. Carers had significantly poorer mental well-being (p = 0.014), measured by their scores on the GHQ-12, compared to non-carers but the difference was very small (η2 = 0.001). At the cross-sectional level, three predictors were identified to most strongly impact mental well-being: subjective financial status; satisfaction with leisure time; and employment status (partial eta squared = 0.053; 0.046; 0.051 respectively). Longitudinal analysis suggested that carers’ mental well-being was poorest prior to undertaking caring. However, there was no evidence that mental well-being became worse for those that became carers relative to those who remained non-carers. Middle-aged male carers who were under financial stress, unable to pursue leisure activities and unemployed or long term sick were found to be particularly vulnerable to poorer mental well-being. Providing support to this group of middle-aged men both in practical and therapeutic terms would help meet their mental well-being needs.