Three MOOCs for Educators (MOOC-Eds) were designed for mathematics and statistics teachers based on principles of effective online professional development that include: self-directed learning, learning from multiple voices, job-connected learning, and peer-supported learning (Kleiman et al., in: Kim (ed) Massive Open Online Courses: the MOOC Revolution, Routledge, New York, 2015). We examined how these design principles were enacted in the development of the MOOC-Eds and how they influenced the engagement of 5767 participants. We also analyzed opportunities the MOOC-Eds provided for participants to develop their knowledge of, beliefs about, and attitudes towards teaching mathematics and statistics. The Interconnected Model of Professional Growth (Clarke and Hollingsworth in "Teach Teach Educ" 18(8):947-967, 2002) was used to guide analysis of click data, discussion forum posts, and interviews to consider the ways in which elements of the external domain influenced the personal domain of participants. Evidence is presented to illustrate the enactment of design principles and their effects.