The 4-H Youth Development Program can help address low levels of scientific literacy among K-12 youth in the United States by providing opportunities to learn science in out-of-school settings. To help ensure quality program delivery, effective professional development for adult volunteers who serve as 4-H science educators is essential. Lesson study, a constructivist-based professional development model, is one potential strategy to help meet this need. A sequential explanatory mixed-methods design was used to investigate the influence of lesson study on 4-H volunteers' science content and pedagogical knowledge. In mixed-methods research, both quantitative and qualitative data are collected and analyzed in an investigation. Survey data revealed improved understanding and use of subject matter knowledge among participants. Focus group interview data elaborated on participants' understanding and use of inquiry processes. Results from this study could benefit 4-H volunteers, other nonformal educators, and researchers.