Traditionally, in attachment theory, secure attachment has been linked to parameters of mental health, while insecure attachment has been associated with parameters of psychopathology. Furthermore, spirituality and attachment to God have been discussed as corresponding to, or compensating for, primary attachment experiences. Accordingly, they may contribute to mental health or to mental illness. In this cross-sectional observational study, we investigate attachment styles (Avoidant and Anxious Attachment; ECR-RD), spirituality (Religious and Existential Well-Being; MI-RSWB), and mood pathology (Anxiety, Depression, Somatization; BSI-18) in 481 (76% female) young adults (age range: 18–30 years) who had a Roman Catholic upbringing. In accordance with previous research, we found insecure attachment to be associated with low levels of spirituality. Furthermore, insecure attachment and low levels of spirituality were associated with higher levels of mood pathology. In hierarchical regression analyses, only Anxious Attachment positively predicted all three dimensions of mood pathology while Existential Well-Being – but not Religious Well-Being–was an additional negative predictor for Depression. Our results underline that spirituality can correspond to the attachment style, or may also compensate for insecure attachment. Higher Existential Well-Being–comprised of facets such as hope for a better future, forgiveness and the experience of sense and meaning–seems to have an especially corrective effect on mood pathology, independent of attachment styles. Our findings emphasize the vital role of existential well-being in young adults’ affective functioning, which might be considered in prevention and treatment. Further research in clinical surroundings is recommended.