Elevated mindfulness predicts lower obsessive-compulsive disorder (OCD) and generalized anxiety disorder (GAD) symptoms. However, the majority of existing research is based on self-report measures and laboratory studies. In the current study, we examined whether attentional control, measured naturalistically, moderates the relationship between trait mindfulness and obsessive-compulsive (OC) and generalized anxiety symptoms. Participants completed a smartphone-based flanker task to index attentional control in their natural environments as well as self-report measures of trait mindfulness, anxiety, and OC symptoms. Results revealed that greater trait mindfulness predicted lower OC and anxiety symptoms only when attentional control, measured naturalistically, was high. These results suggest that the inverse relationship between trait mindfulness and symptoms of OCD and GAD depends on the ability to sustain attention. Thus, attentional control may be a shared moderator through which higher trait mindfulness impacts symptoms. Efforts to enhance attentional control in naturalistic settings may therefore improve the efficacy of treatments for OCD and GAD that rely on mindfulness.