Purpose: To evaluate the effectiveness of in-person versus online Girl Scout leader wellness training for implementation of wellness-promoting practices during troop meetings (phase I) and to assess training adoption and current practices across the council (phase II). Design: Pragmatic superiority trial (phase 1) followed by serial cross-sectional study (phase II). Setting: Girl Scout troop meetings in Northeast Kansas. Participants: Eighteen troop leaders from 3 counties (phase 1); 113 troop leaders from 7 counties (phase II). Intervention: Phase I: Troop leaders attended 2 wellness training sessions (first in groups, second individually), wherein leaders set wellness-promoting practice implementation goals, self-monitored progress, and received guidance and resources for implementation. Leaders received the intervention in person or online. Measures: Phase I: At baseline and postintervention, leaders completed a wellness-promoting practice implementation questionnaire assessing practices during troop meetings (max score = 11). Phase II: Leaders completed a survey about typical troop practices and interest in further training. Analysis: Phase I: Generalized linear mixed modeling. Results: Phase I: In-person training increased wellness-promoting practice implementation more than online training (in person = 2.1 ± 1.8; online = 0.2 ± 1.2; P = .022). Phase II: Fifty-six percent of leaders adopted the training. For 8 of 11 wellness categories, greater than 50% of leaders employed wellness-promoting practices. Conclusion: In-person training was superior to online training for improvements in wellness-promoting practices. Wellness training was adopted by the majority of leaders across the council.