This study assesses the mental well-being of young adults (18-25 year olds), a recognized weight gain time period, in relation to self-reported weight, diet, physical activity and other risky lifestyle behaviours. A questionnaire survey was conducted amongst young adults in the North-East of Scotland. Mental well-being was assessed using the Warwick-Edinburgh Mental Well-being Scale. Demographic, diet, physical activity, smoking, alcohol, sexual relations and drug taking factors were investigated. Univariate analyses and generalised linear models explored the most informative factors with respect to mental well-being. One thousand one hundred and thirteen young adults responded. Lower mental well-being scores were associated with being underweight, those suffering from obesity, snacking habits and for drug takers. Increased physical activity (PA) was linked with better mental well-being even if PA was not enjoyed. Improved mental well-being was also associated with having or having had a sexual partner. Education, smoking and alcohol provided no additional information. This large cross community study utilised different factors altogether and consequently provides important information on emerging adults. The results indicate that adaptable behaviours (body weight, nutrition, exercise, personal relationships, attitudes towards drugs) affect mental well-being. Future interventions should consider these lifestyles and risky behaviours to promote not only future health but also positive mental health of this often neglected, vulnerable age group.