A great opportunity for mobile devices to transform the way students learn by transforming conventional classrooms into more interactive and engaging environments. But using mobile for learning online made various challenges. Online discussion has been more popular in asynchronous, blended learning, and mainly distance learning courses. Online discussion aids in the development of a learning community, enables information exchange, and promotes the development of High Order Thinking Skills, it can be achieved with critical thinking, giving complex problems in learning, dilemmas in unfamiliar questions, and arguments and comparisons in learning activities. Online discussion is a solution in building high-order thinking skills. However, there is a lack of engagement and motivation when students using online discussion. This study used mixed-methods that placed a focus on qualitative and quantitative techniques and design-based research to build a mobile prototype online discussion using the Andragogy principle and gamification elements to enhance student engagement. The result of this research was 73% for the score of the user engagement scale. This indicates that the proposed prototype could improve user engagement.