Mesoporous carbons attract increasing attention owing to their potential applications in supercapacitors. So far, controlled synthesis of mesoporous carbons with a narrow pore size distribution relies largely on the complicated template methods. To avoid the use of templates, a surfactant–free emulsion polymerization method is presented for the fabrication of a melamine–modified phenolic resin microrod (MPRR) assembled by micron–sized spherical cells and thin walls. In addition, one–step KOH activation strategy is adopted to synthesize hierarchical mesoporous activated carbon with 2–10 nm narrow mesopores by using MPRR as carbon precursors. The as–prepared mesoporous activated carbon has a high specific surface area of about 2758 m2 g−1 and a mesopore volume of 0.54 cm3 g−1 (calculated by density functional theory), comprising ∼43.5% of total pore volume (∼1.43 cm3 g−1). Hierarchical mesopores can significantly accelerate ion transfer and increase micropore accessibility, which endow the carbon with high specific capacitance equal to 409 F g−1 at 0.1 A g−1 and 268 F g−1 at 100 A g−1 in 6 M KOH electrolyte, with a high capacitance retention of 66%. Moreover, the assembled symmetric supercapacitor also exhibits good cycling stability in KOH electrolyte and delivers high power density equal to 12080 W kg−1 when energy density is 5.02 Wh kg−1. This finding provides an insight into directional tailoring of mesoporous structures of phenolic resin–based carbon materials at the molecular level for high–performance supercapacitors.