Massive Open Online Courses (MOOCs) provide borderless opportunities to engage with content and ideas, with prospective participants from around the globe being able to easily register. The course featured in this study focused on the social and emotional well-being of adolescents, selected because of a recognized need for professional learning related to this topic. It was purposely designed for delivery as a MOOC and was designed as a 6h program around four topics to be completed over two weeks in asynchronous mode. It was delivered seven times from 2019 to 2023, with 32,969 individuals enrolled across these deliveries. The effectiveness of engaging in the course for professional learning purposes is of particular interest to this study. To that end, a convergent mixed methods study was conducted. First, quantitative and qualitative survey data collected at various course stages were examined to reveal the demographic characteristics of participants and their experiences in the course using data from surveys and comments about their experiences. The findings revealed, among other factors, that 65% were female, with just under half (47%) aged 45 years or less, nearly half (44%) held a bachelor’s degree as their highest level of qualification, and 48% were employed within the teaching and education sector. The most active learners were from Europe (48%) and Asia (27%), with active learners from a total of 178 countries. The course has a high course retention index, with 51% of learners completing 51% of the course and 8383 learners completing 90% or more of the course. The qualitative findings reveal the strongly positive experiences reported by the active participants. Secondly, we examined the effectiveness of the MOOC for participants’ professional learning needs by assessing the course using a framework with ten domains related to its core design features, modified for use by the course designers as a self-reflective tool. We found that the domains that scored the lowest were collaboration, interactivity, and, to a lesser extent, pedagogy. The study’s limitations include the incomplete data provided as part of the MOOC protocols, and the use of a self-reflection tool, which may inadvertently incorporate bias. This study points to these gaps in the data, including the need to access longitudinal data that go beyond a focus on the design of courses to extend to the impact and outcomes of the experience.