Objectives <p>Mindfulness-based digital interventions have shown promise for improving health outcomes. The Mindfulness Coach (MC) mobile app was found to improve posttraumatic stress disorder (PTSD) and depression symptoms in a pilot randomized controlled trial (RCT). The current secondary analyses expand on these findings by assessing changes in mindfulness-related processes targeted by the app and by qualitatively exploring users’ experiences to better understand how Mindfulness Coach may support these processes in the context of PTSD.</p> Method <p>U.S. veterans with PTSD (<i>n</i> = 173) were randomized to an 8-week MC (<i>n</i> = 86) or waitlist control (WL; <i>n</i> = 87) condition. After study attrition due to technical issues and participant dropout, 56 participants (MC: <i>n</i> = 19; WL: <i>n</i> = 37) completed follow-up surveys. A secondary quantitative intent-to-treat analysis was conducted on process outcomes related to facets of mindfulness, psychological flexibility, interoceptive awareness, and thought suppression using linear mixed modeling with maximum likelihood estimation for missing values. A Rapid Qualitative Analysis of 1-year follow-up interviews with a subsample of participants in the MC condition (<i>n</i> = 21) examined PTSD-related coping responses, knowledge and experiences related to mindfulness practice prior to the study, as well as use and perceptions of Mindfulness Coach and its potential impact.</p> Results <p>Among study completers (MC: <i>n</i> = 19; WL: <i>n</i> = 37), there were significant between-group effects for two facets of mindfulness (i.e., describing and nonjudgment; <i>p</i>-values &lt; 0.05). Significant within-group effects for both conditions were also found for psychological flexibility (<i>p</i>-values &lt; 0.05). Qualitative results revealed mostly positive user experiences, potential barriers to use, and perceived improvements in stress, PTSD symptoms, and sleep.</p> Conclusions <p>This study showed improvements in mindfulness processes targeted by Mindfulness Coach and perceived mental health benefits with app use, supporting its value as an on-demand resource for veterans with PTSD. High attrition rates limit the strength and generalizability of these findings and underscore the challenges of sustaining engagement with self-guided digital interventions. However, this low-cost and scalable tool holds promise as an option for veterans with PTSD that can extend the reach of evidence-based mindfulness strategies to improve mental health care.</p> Preregistration <p>ClinicalTrials.gov, TRN: NCT04035304, Registration date: 24 July 2019.</p>