Many women experience depression and/or anxiety during pregnancy and/or the postpartum period. Mind-Body Interventions (MBIs) have shown great efficacy in the mitigation of these symptoms; however, there is limited research spanning the postpartum period and exploring long-term sustainability of mindfulness practice. Furthermore, little is known about specific facilitators that contribute to sustainability. The primary objective of this mixed-methods study is to use the Theory of Planned Behavior (TPB) framework to inform multi-level factors associated with the sustainability of mindfulness practices and associated mental health outcomes in the postpartum period after participating in a prenatal MBI to inform and improve future interventions. Measures of mental health including depression, anxiety, and perceived stress were evaluated via quantitative assessments at three time points (n = 24) across the prenatal and postpartum periods prior to and following the prenatal MBI. Interviews were conducted post-intervention (n = 10) and 5-12-months postpartum (n = 8) to identify sustainability facilitators. Measures of depression, anxiety, and perceived stress were significantly lower post-intervention than pre-intervention, and decreased levels of depression were sustained over three months postpartum. After participation, all interview participants reported intention to continue practice postpartum. In the postpartum period, 73% of participants reported weekly sustained practice. Qualitative interviews revealed significant facilitators to be informal practice (attitudes and beliefs), family and peer support (social norms), life-integration and self-compassion (perceived control) and benefits to child (intentions/ behaviors). These findings suggest that future interventions should focus on ease of integrating informal practice into daily routines, communication with support systems, self-compassion, and benefits of mindfulness practices for the child to promote sustainability.