A collaborative approach to online learning design where educators partner with interdisciplinary digital learning professionals has emerged as a key strategy for creating high-quality online learning environments. Beyond its pedagogical benefits, collaborative design is increasingly seen as an authentic form of professional development for educators. While this claim has a strong theoretical foundation, empirical evidence remains limited. This study addresses that gap by exploring how inter-disciplinary collaborative design experiences contribute to educators’ learning and practice. The research is grounded in Engeström’s expansive learning theory, Kalantzis and Cope’s concept of learning by design, and McKenney et al’.s ecological framework on educators’ design knowledge. A qualitative multiple case study was employed, with data collected through two phases of one-to-one semi-structured interviews (n = 29) and observational data from six design teams across five UK universities. There are three main findings: first, educators developed a (co-)design mindset and associated capabilities; second, they experienced shifts in their pedagogical thinking and approaches; and third, they expanded their awareness and use of learning technologies. This article highlights the value of purposeful collaborative design as a vehicle for integrated professional development and provides a framework outlining key dimensions of educator development essential for high-quality online learning design.