COVID-19 long haulers face profound psychosocial stressors (e.g., depression, anxiety, PTSD) and physical health challenges (e.g., brain fog, fatigue). This study tests the feasibility and initial impact of a digitally delivered mindful-walking (MW) intervention for improving the physical and psychosocial wellbeing of COVID-19 long haulers. We recruited 23 participants via Facebook groups, between March and November 2021, for a 4-week online MW intervention (i.e., 2 mindfulness practice sessions per week), that was delivered entirely through the study Facebook group. The intervention was assessed using mixed methods. Quantitative data were collected through brief daily evening surveys (i.e., 28 days) over the 4-week intervention period, and measured affect, cognition, mindfulness, physical activity, and MW engagement. Qualitative data were extracted from the Facebook group's Paradata (i.e., participant feedback, engagement metrics, and all social media interactions). Multilevel modeling was employed for the statistical analysis and a pragmatic approach was used for the qualitative analysis. The participants reported a high feasibility score (mean=4.93/7, SD=1.88), which was comprised of perceived usefulness, satisfaction, and ease of use. Those who engaged in MW, on any given day, frequently reported better psychosocial moods with more positive affect (β=0.89, p<0.01), less negative affect (β=-0.83, p<0.01), higher perceived cognitive ability (β=0.52, p<0.05), and more physical activity (β=0.41, p<0.05). Additionally, participants who practiced MW more consistently during the study reported higher levels of momentary mindfulness (β=0.3 p<0.01). Participants expressed satisfaction with the intervention, reporting benefits such as better symptom management and an overall improvement in wellbeing. Despite the small sample size, the digital delivery of our MW intervention via Facebook showed high acceptability. Preliminary efficacy findings indicate improved mental wellbeing and physical activity among long haulers. Larger-scale RCTs are needed in the future to improve the robustness and applicability of findings.