BACKGROUND Adult children of parents with mental illness experience a myriad of complex emotions as they attempt to make meaning of the lived experiences of their parents. A crucial time for adult children is emerging adulthood, a time when they move away from their family of origin and establish their own identity and independence. Despite existing research that provides a static description of adult children's lived experiences, the literature lacks an explanatory theory about the dynamic, relational processes that occur as adult children progress from one life stage to the next. METHODS The current study aimed to develop an explanatory theory of the relational trajectory that adult children might experience as they course through adulthood and parenthood over time. Semistructured interviews using grounded theory analysis were conducted with 10 adult children aged between 27 and 51 years old. RESULTS Three key phases within the Relational Trajectory Model (RTM) were identified: (i) confusion, (ii) contemplation, and (iii) reconciliation. By reflecting on their own parenting role, adult children were able to reach an evolved parental identity, with the majority of participants also making relationship reparations with their parents with mental illness. Parallels are drawn to theories of identity and intergenerational family systems to further explain and substantiate the processes encompassed within the RTM. CONCLUSIONS Generating an explanatory theory serves as a potential guide for mental health professionals working with families with parental mental illness, by drawing attention to the intricacies of familial relationships and interpersonal ties.