In this work, we propose a general template-free strategy for fabricating two-dimensional mesoporous mixed oxide nanosheets, such as metal cobaltites (MCo2O4, M = Ni, Zn) through the self-deconstruction/reconstruction of highly uniform Co-based metal glycerate nanospheres into 2D Co-based metal glycerate/hydroxide nanosheets, induced by the so-called "water treatment" process at room temperature followed by their calcination in air at 260 °C. The proposed 'self-deconstruction/reconstruction' strategy is highly advantageous as the resulting 2D metal cobaltite nanosheets possess very high surface areas (150-200 m2 g-1) and mesoporous features with narrow pore size distribution. In addition, our proposed method also enables the crystallization temperature to achieve pure metal cobaltite phase from the precursor phase to be lowered by 50 °C. Using the 2D mesoporous NiCo2O4 nanosheets as a representative sample, we found that they exhibit 6-20 times higher specific capacitance and greatly enhanced capacitance retention compared to the NiCo2O4 nanospheres achieved through the direct calcination of the Ni-Co glycerate nanospheres. This highlights another advantage of the proposed strategy for enhancing the electrochemical performance of the mixed oxide products for supercapacitor applications. Furthermore, the asymmetric supercapacitor (ASC) assembled using the 2D NiCo2O4 nanosheets//graphene oxide (GO) exhibits a maximum energy density of 38.53 W h kg-1, while also showing a high capacitance retention of 91% after 2000 cycles at 5 A g-1. It is expected that the proposed general method may be extended to other transition metal elements for creating 2D mixed oxide nanosheets with enhanced surface areas and improved electrochemical performance.