The Project Youth Extension Service (YES!) college student internship integrates pre-service training on youth worker competencies with a year or more of practice experience in leading positive youth development programs for military youth impacted by the military deployment process. For nearly a decade, interns have reported significant improvement in 37 behavioral competencies, with 24 indicators improving .50 or more on a 5-point scale. Areas of greatest growth include practices critical to youth worker effectiveness and program quality: self-regulation, interaction, and adaptation in high-intensity settings. Qualitative comments indicate growth in personal maturity (e.g., composure, flexibility, openness to feedback) as well as professional growth (e.g., listening, organization, presentation, teamwork), and empathy for youth and families under stress. Intern growth was also observed by trainers and mentors. Program feedback from youth and event coordinators was consistently positive. Interns also benefited from working with a career mentor. Blended online and on-site training, episodic scheduling, and scaffolded leadership offer effective and efficient methods for programming and professional development. Findings point to the value of intensive and extended training focused on experiential learning, critical reflection, and mentoring, together with background knowledge on military culture, military family life, and youth development. The program model, full results, conclusions, recommendations for practice, and opportunities for improvement are discussed.