The present study aims to investigate explicit and implicit affective attitudes toward sustainability, aspects of mindfulness, and their related factors. Therefore, different areas of sustainability (e.g., mobility, nutrition, packaging) were considered. We focused mainly on the indirect effects of mindfulness on attitudes through prosocialness and connectedness to nature. Furthermore, heartfulness, as the emotional quality of mindfulness, was considered. 458 subjects answered a series of questionnaires (mindfulness, connectedness to nature, prosocial behavior, gratitude, and self-compassion questionnaire) and completed an explicit affective evaluation task, as well as an affective priming task measuring implicit attitudes. The participants explicitly evaluated sustainable concepts more positively in comparison to non-sustainable ones. There was no significant correlation between implicit and explicit affective attitudes. Furthermore, the results of structural equation modeling revealed inner awareness and insight as predictors for prosocialness, and prosocialness in turn for the explicit affective attitudes toward sustainability. In addition, exploratory analyses showed a significant relationship between self-compassion and explicit attitudes toward sustainable concepts and a significant overall relation between gratitude, prosocialness, and explicit attitudes toward sustainable concepts. None of the relations to the implicit attitudes was significant. Our findings suggest prosocialness as a relevant mediating factor for the relation of explicit attitudes toward sustainability and mindfulness. However, this connection might be more complex, and the differentiation of mindfulness aspects inner and outer awareness seems insightful and should be considered even more in further research.