Depression is a serious mental health issue, affecting an increasing number of people, particularly students. Although effective treatments exist, relapse and remission rates remain high, emphasising the need to better understand the underlying mechanisms. This thesis examined how cognitive and trait vulnerability factors, together with daily behaviours contribute to mental health and brain health across five empirical studies. Study 1 (cross-sectional, N = 546), examined mediators of the association between chronotype and depressive symptoms, showing that chronotype was not directly related to depressive symptoms; instead, acting with awareness, describing, sleep quality, and alcohol use mediated this relationship. Study 2 was a longitudinal study and demonstrated that within-person reductions in quality of sleep and acting with awareness predicted increases in depressive symptoms, while increases in rumination were linked to decreases in non-judgement, non-reactivity, and acting with awareness facets of trait mindfulness. As a significant predictor of depressive symptoms in previous studies, Study 3 found that neuroticism moderated the association between rumination and sleep quality longitudinally, but not cross-sectionally. Neuroimaging studies provided further evidence by showing how cognitive and trait vulnerability factors affected brain health. Study 4 revealed that total mindfulness, acting with awareness, and observing facets might play a role in preserving hippocampal volume in healthy students. Finally, Study 5 showed that higher non-reactivity scores were associated with younger-appearing brains in mid-age healthy adults in exploratory analyses. Although, the majority of these neuroimaging results did not survive after correcting for multiple comparisons, they offer important insights as there is limited research which investigates brain health using the FFMQ in healthy adult samples. Together, these findings emphasise the need to target modifiable risk and vulnerability factors to promote mental and brain health.