Herein, we show that soft-templating synthesis of biosourced OMCs under rather mild pH conditions is possible by controlling the reaction temperature, without using either formaldehyde or toxic phenolic reagents. Removal of the surfactant at 400 °C and subsequent carbonisation at 900 °C allows reducing the shrinkage and preserving the material mesoporosity. Self-assembly of mimosa tannin with Pluronic® F127 in mild conditions (30 °C, pH 3) leads to hierarchical OMCs with more developed porosities, more ordered mesostructures and higher nanotexture ordering than in the more acidic conditions that are commonly used in soft-templating of OMCs. Further development of the OMC porosity by physical activation with CO2 increased the micropore volume by ∼60% without significantly affecting the structural ordering. However, it decreased the mesopore volume and the oxygen content and the PSD was slightly widened in the mesopore range. The activated OMC exhibited high gravimetric capacitances of 286 F g−1 (determined by cyclic voltammetry), a high capacitance retention of 50% at 12 A g−1 (determined by galvanostatic charge-discharge), high energy and power densities of 5.41 W kg−1 and 5.03 kW kg−1, respectively, and a faster frequency response, 4.39 s, than the non-activated OMC, making it a more appropriate material for practical supercapacitors.