Comorbid psychiatric presentations, defined as those who present with more than one mental and/or behavioral health diagnosis at the same time, during adolescence are on the rise. Mindfulness-based interventions can alleviate psychological symptoms and improve emotion regulation in youth. Mindfulness is a multifaceted phenomenon, with five underlying facets (Observing, Describing, Acting with Awareness, Non-Judgment and Non-Reactivity of Inner Experience). Little evidence has documented which facets produce pronounced psychiatric symptom reduction for adolescents. This pilot study examined the efficacy of an online mindfulness-based intervention delivered to adolescents undergoing mental health treatment during COVID-19 to reduce psychiatric outcomes. Fifty-six adolescents (m = 14.5 years, 66.1% female) categorized as moderate-risk (treatment histories of outpatient therapy only) or high-risk (treatment histories with intensive service participation) participated in the 8-session mindfulness-based intervention. Significant reductions in psychiatric symptoms and increases in adaptive coping strategies were observed at post-test, particularly for those at moderate-risk. Multivariate stepwise regression found significant associations between mindfulness facet use and anxiety, depression, and somatic symptoms (R2 ranging from 42.5 to 52.8%). Results indicate preliminary efficacy for an online mindfulness-based intervention for adolescents, particularly those at moderate-risk, due to the introduction of new coping skills, given their history of less intense treatment. Further investigation is warranted to understand which mindfulness facet intervention components produce the most prominent outcomes.