Biomass-derived porous activated carbons hold great promise for applications in supercapacitors due to their high surface area, low cost, self-heteroatom doping, and stable physicochemical features. However, their electrochemical performance, particularly in aqueous environments, is largely influenced by the nature of the pore structure, which significantly relies on the activation process and the nature of biomass precursor. Here, we present an effective and environmentally friendly strategy for synthesizing nitrogen-doped hierarchical porous carbon materials (NRCs) for supercapacitor applications. In this method, algae-derived bio-oil distillation residue is used as a carbon source in the presence of nano-CaCO3 and K2CO3 as a templating agent and activator, respectively. The effect of carbonization temperature and the stoichiometric ratio between CaCO3 and K2CO3 have been widely investigated. The NRC1:2-800 sample, with a 1:2 ratio of CaCO3 to K2CO3 at a carbonization temperature of 800 °C, shows the highest surface area (2376.95 m2 g−1) among other samples, reflecting the highest specific capacitance either (352.5 F g−1). The material further shows a maximum energy density of 15.9 Wh kg−1 at 400 W kg−1 for the symmetric device measured in 1 M Na2SO4. Finally, the effect of pore size on the electrochemical performance of the prepared activated carbons was investigated via theoretical calculations, indicating that a pore size of 2–5 nm is more favorable for ion transport and thus better improves the electrochemical performance of activated carbons. This study thus lays a solid foundation for creating highly efficient and porous activated carbon for energy storage devices.