This study endeavours to investigate the mediating role of synergy mentality between strategic planning relationship and a firm's performance. The study used the descriptive research design and the conceptual framework was tested using a multiple regression model. Data were collected using questionnaires distributed to senior managers, general managers and directors of manufacturing companies in Indonesia. The hypotheses were analysed using IBM SPSS. The results suggest that not only does strategic planning have a direct impact on a firm's performance but also on the mediating role of synergy mentality. In support of past findings, the present study shows that strategic planning has a positive relationship related to a firm's performance especially with synergy mentality within a firm's managerial levels. The present study is one of the first to empirically examine and confirm the mediating role of synergy mentality on strategic planning and firm performance.