In recent years, studies have discussed how to introduce computational thinking (CT) concepts in mathematics education through mobile app development. In this study, the design of mathematics lessons based on the 5E instructional model to extend the idea of CT in a mobile technology environment (i.e., mobile CT) was investigated. Twenty-three primary five students in Hong Kong participated in this study. The teacher taught the students how to develop a mobile calculation game to learn the mathematical concept "area" through paper prototyping and mobile app development activities. Using a design-based research approach, the study examined students' performance and behavior in the classroom to acquire mathematics knowledge and mobile CT. Qualitative conversation analysis was used to interpret teacher-student interaction, code files, and screen captures of students' work. The analysis provided evidence on how students constructed mathematics concepts about "area" and built their mobile calculation games using mobile CT concepts, practices, and perspectives. The results propose the use of the 5E instructional model to enhance students' engagement in and motivation for mathematics learning and strengthen their problem-solving skills, critical thinking, and communication and collaboration skills. Mobile CT-integrated mathematics lessons suggest ways for future educators to incorporate other mathematics topics into CT education. This study recommends that the 5E instructional model could be suitable for the instructional design of primary school CT-integrated mathematics curriculum. A set of design principles for integrating CT into mathematics curriculum is recommended.