Online training is feasible, but the impact of brief mindfulness training on health professionals needs to be better understood. We analyzed data from health professionals and trainees who completed self-reflection exercises embedded in online mindfulness training between May 2014 and September, 2015; their changes in mindfulness were measured using standardized scales. Participants included nurses (34%), physicians (24%), social workers and psychologists (10%), dietitians (8%), and others (25%); 85% were women, and 20% were trainees. The most popular module was Introduction to Mindfulness (n = 161), followed by Mindfulness in Daily Life (n = 146), and Mindful Breathing and Walking (n = 129); most (68%) participants who took 1 module took all 3 modules. There were no differences in participation in any module by gender, trainee status, or profession. Completing modules was associated with small but significant improvements on the Cognitive and Affective Mindfulness Scale-Revised, the Mindful Attention Awareness Scale, and the Five Facet Mindfulness Questionnaire (P < 0.001 for all). Online training reaches diverse health professionals and is associated with immediate improvements in mindfulness. Additional research is warranted to compare the long-term cost-effectiveness of different doses of online and in-person mindfulness training on clinician burnout and quality of care.