Questionnaire-based approaches for assessing mindfulness are well-established in research but have notable methodological limitations, particularly regarding validity. The Mindful Breathing Exercise (MBE), an experimental task, offers a complementary approach to rating scale assessments. Modeled on traditional breathing meditations, such as those used in Mindfulness-Based Stress Reduction, the MBE measures breath awareness during an 18-min computer-based meditation practice. Participants are instructed to focus on their breath, while acoustic signals are played at irregular intervals. At each signal, participants report whether they are still focused on their breath or if their mind has wandered. In addition, they are asked to indicate whenever they notice their mind wandering between signals. A high MBE score indicates fewer instances of mind wandering during the task. The limited research on its psychometric qualities suggests the MBE has reasonable convergent and discriminant validity.