Adults play important roles in the lives of adolescents, serving as role models, sources of social support, and providers of social capital. Natural mentoring relationships (NMRs), relationships between adults and youth that rise to a level of significance for the youth, have a positive impact on youth outcomes in a number of academic, socioemotional, and health related areas. Yet despite the importance of NMRs to youth development, there is little research on how NMRs develop or what factors are associated with a relationship becoming an NMR. This study uses in-depth qualitative interviews with adolescents across five time points to explore the characteristics of youth-adult relationships that develop into NMRs, and the psychosocial processes and actions which transform a naturally occurring relationship into an NMR. Findings from our study suggest that by intentionally using time and space with youth as an opportunity to cultivate safe and authentic spaces and build trust, adults can encourage the development of NMRs with adolescents. [This paper was published in "Children & Youth Services Review" v111 Article 10487 2020.]