Although the positive outcomes of human–environment interactions have been established, research examining the motivation between engagement in pro-environmental activities and psychological well-being is limited. In this mixed-methods study, the relationship between pro-environmental engagement, meaning in life, and well-being, including loneliness and depression, were investigated in a sample of 112 young adults in Canada. It was found that engaging in pro-environmental activities was negatively associated with loneliness. This association was mediated by meaning in life (e.g., an intrinsic motive of caring for future generations). In addition, qualitative analyses explored how engaging in pro-environmental activities has a meaningful impact on meaning in life, and on well-being. A thematic analysis generated three unique themes: (1) responsibility to teach the next generation about the environment, (2) deep appreciation for and connection to nature, and (3) renewed agency through self-directed learning. Overall, findings suggest that meaning in life is a core motive that underlies the association between environmental engagement and loneliness. The present study enriched the relationship between pro-environmentalism and well-being with a mixed-methods perspective.