The 21st century has brought an increasing demand for expertise in science, technology, engineering, and math (STEM). Although strides have been made towards increasing gender diversity in several of these disciplines, engineering remains primarily male dominated. In response, the U.S. educational system has attempted to make engineering curriculum more engaging, informative, and welcoming to girls. Specifically, project-based and design-based learning pedagogies promise to make engineering interesting and accessible for girls while enculturating them into the world of engineering and scientific inquiry. Outcomes for girls learning in these contexts have been mixed. The purpose of this study was to explore how cultural gender norms are navigated within informal K-12 engineering contexts. We analyzed video of single- and mixed-gender collaborative groups participating in Studio STEM, a design-based, environmentally themed afterschool program that took place in a rural community. Discourse analysis was used to interpret interactional styles within and across groups. Discrepancies were found regarding functional and cultural characteristics of groups based on gender composition. Single-gender groups adhered more closely to social gender norms. For example, the boys group was characterized by overt hierarchies, whereas the girls group outwardly displayed solidarity and collaboration. In contrast, characteristics of interactional styles within mixed gender groups strayed from social gender norms, and stylistic differences across group types were greater for girls than for boys. Learning outcomes indicated that girls learned more in mixed-gender groups. Our results support the use of mixed-gender collaborative learning groups in engineering education yet uncover several challenges. We close with a discussion of implications for practitioners.