Objective: To identify the key mechanisms, contexts, and outcomes that drive the successful participatory co-design of assistive technologies. Method: A rapid realist review was conducted using a systematic search strategy. After screening, a final set of 28 articles were included. Articles were analyzed for evidence relevant to our initial program theory (IPT), and context-mechanism-outcome configurations were developed, resulting in a revised program theory. Results: All 28 articles included were highly relevant to the IPT, and had sufficient detail regarding the process of participatory co-design. The findings of this review highlight several key context-mechanism-outcome configurations as potential patterns in the data under the two dimensions of the evolving program theory: knowledge integration and the ethico-political dimension. Discussion: This review revealed the key mechanisms of mutual awareness, mutual learning, trust, and reciprocity that need to be taken into account in AT development and assessment. We concluded that participatory co-design requires a restructuring of power relations between end-users and those traditionally in control of technology design. These findings inform the development and assessment of AT for older adults and help guide policy/decision-makers to move forward with the now urgent agenda for scale-up and spread, initiated by the burning platform of the COVID-19 pandemic.