High mass loading and fast charge transport are two crucial but often mutually exclusive characteristics of pseudocapacitors. On conventional carbon supports, high mass loadings inevitably lead to sluggish electron conduction and ion diffusion due to thick pseudocapacitive layers and clogged pores. Here we present a design principle of carbon supports, utilizing self-assembly and microphase-separation of block copolymers. We synthesize porous carbon fibers (PCFs) with uniform mesopores of 11.7 nm, which are partially filled with MnO of <2 nm in thickness. The uniform mesopores and ultrathin MnO enable fast electron/ion transport comparable to electrical-double-layer-capacitive carbons. At mass loadings approaching 7 mg cm , the gravimetric and areal capacitances of MnO (~50% of total mass) reach 1148 F g and 3141 mF cm , respectively. Our MnO -coated PCFs outperform other MnO -based electrodes at similar loadings, highlighting the great promise of block copolymers for designing PCF supports for electrochemical applications.