Background Weight-loss approaches involving mindfulness have been reported to reduce overeating behavior. We conducted a preliminary evaluation of the feasibility and effectiveness of a mindfulness mobile application (MMA) combined with a comprehensive lifestyle intervention (CLI) focused on weight loss and eating behaviors for people with metabolic syndrome based on post-intervention follow-up data. Method Participants were randomly assigned (1:1) to a CLI group or a CLI + MMA group. Participants received weekly CLI for 13 weeks, followed by telephone counseling for 13 weeks. The CLI + MMA group also had access to the MMA. Feasibility was assessed by the number of people who refused to participate, rate of adherence to the MMA, follow-up rate, and participant satisfaction. The preliminary endpoint was weight change (at 26 weeks). Participants completed the Dutch Eating Behavior Questionnaire (DEBQ). A mixed linear model was used for efficacy analysis. Results Eight of the 40 participants declined to participate. The MMA was used 4.4 ± 1.7 days per week, but the rate of adherence declined over time. The follow-up rate was 100%, and there was no difference in participant satisfaction between the groups. There was no significant group-by-time interaction for weight loss (p = 0.924), but there was a significant interaction for the DEBQ restrained eating score (p = 0.033). Conclusions This study found that CLI plus MMA was highly feasible and moderately acceptable. There were no significant differences in weight loss between the groups, but the CLI + MMA group showed an increase in restrained eating. Further large-scale studies are needed. Trial Registration Japanese University Hospital Medical Information Network (UMIN-ICDR). Clinical Trial identifier number UMIN000042626.