Transitioning from pregnancy to parenthood is particularly challenging for women living with low income and experiencing social isolation, mental illness, addiction, and/or family violence. The purpose of this qualitative study was to evaluate one component of Welcome to Parenthood, a two-generation multiple intervention program including neuroscience-based parenting education, kin and non-kin mentorship, and an engagement tool (baby kit). From late pregnancy to 2 months postpartum, mentors kept a journal regarding their experiences of mentoring mothers experiencing vulnerability. We engaged in a modified constructivist grounded theory to explore hand-written text from the journals. The core category, Struggling with Reciprocity and Compassion, influenced processes of Becoming a Mentor. Mentoring mothers experiencing vulnerability was both challenging and rewarding, requiring an inordinate amount of physical, social, emotional, and economic resources. To foster maternal mental health and infant development, pregnant and parenting women experiencing vulnerability could benefit from long-term reciprocal and compassionate mentoring.