This study aims to investigate the effect of history-of-mathematics activities on the development of mental computation of middle-grade students and to determine the students' views on the history of mathematics. This study was carried out as an action research project. The participants of the study consisted of 19 seventh-grade students studying at a middle school and a mathematics teacher. A mental computation proficiency test, a questionnaire, semi-structured interviews, field notes taken by the teacher and work sheets for the history of mathematics that were administered to the students were used during the data collection process of this study. At the end of the study, it was seen that the history-of-mathematics activities were effective in some of mathematical operations but not in some others. The history-of-mathematics activities were also found to contribute to the enrichment of computational strategies of the middle graders. Moreover, the middle grade students and the teacher who participated in the study stated that the history-of-mathematics activities had positive aspects, such as computational practicality, building positive attitudes towards mathematics and transferability to daily life. However, the history-of-mathematics activities were described to be difficult and complex by some of the students. [This study was presented in the 'International Conference on Mathematics and Mathematics Educations' (ICMME-2019) (11--13 July 2019, Konya).]