Mental toughness describes a set of positive psychological resources that predict a range of outcomes, including wellbeing. It has been conceptualized as a state, but research has not yet examined the impact of positive or adverse childhood experiences on its development. Such experiences are known to be important for wellbeing and mental health. The current study aimed to examine the relationships between adverse and positive childhood experiences and later mental toughness, and whether mental toughness mediated the relationship between childhood experiences and wellbeing. Three hundred and eleven participants (53 males and 256 females, with a mean of 35.5 years (SD 13.67) completed measures of childhood experiences, mental toughness, and wellbeing. Positive, but not adverse childhood experiences predicted mental toughness, and there was a significant indirect path from positive experiences to wellbeing via mental toughness. The results are discussed in terms of implications for the theory of mental toughness, and implications for supporting the development of mental toughness, which may in turn enhance wellbeing.