Background: Elevated mental illness prevalence complicates efforts designed to address the opioid crisis in Appalachia. The recovery community acknowledges that loneliness impacts mood and engagement in care factors; however, the predictive relationship between loneliness and retention in medication-assisted outpatient treatment programs has not been explored. Our objectives were to identify associations between mental health factors and retention in treatment and elucidate treatment retention odds. Data were collected from eighty participants (n = 57 retained, n = 23 not retained) of a mindfulness-based relapse prevention (MBRP) intervention for individuals receiving medication for opioid use disorder (MOUD) in Appalachia. Loneliness, depression, and anxiety did not differ between the retained and not retained, nor did they predict not being retained; however, mindfulness was significantly lower among those not retained in treatment compared to those retained (OR = 0.956, 95% CI (0.912–1.00), and p < 0.05). Preliminary findings provide evidence for mindfulness training integration as part of effective treatment, with aims to further elucidate the effectiveness of mindfulness therapies on symptom reduction in co-occurring mental health disorders, loneliness, and MOUD treatment retention.