Background Blended learning approaches offer a promising strategy to address limited access to competency-based psychotherapy training in low-resource settings. This study describes the design and pilot evaluation of a mechanism-informed blended training programme to build the capacity of non-specialist health workers to deliver Group Support Psychotherapy (GSP). Methods We conducted a mixed-methods study comprising two phases: (1) the design and development of the mechanism-informed SEEK-GSP Academy, a blended Learning Management System integrating self-paced online learning, live interactive teaching, supervised practical demonstrations, competency assessment, and implementation of GSP; and (2) a single-arm pilot evaluation among non-specialist health workers and lay providers in Uganda. Quantitative data included participant progression through the training pathway, platform utilization, module ratings, pre- and post-training knowledge assessments, and initiation of GSP delivery. Factors independently associated with knowledge gain and implementation of GSP groups were identified using multivariable linear and logistic regression models, respectively. Qualitative data on participant experiences, platform usability, and implementation challenges were collected through open-ended survey questions and analysed thematically. Results Of 203 trainees enrolled, 157 (77.3%) completed the online training. Among completers, mean knowledge scores increased from 80.1 (SD 16.6) to 91.6 (SD 7.4), a mean gain of 11.5 points (p