The nanostructured phenolic thermosets were prepared from the ternary mixtures composed of novolac resin, poly(styrene-alt-maleic anhydride)-block-polystyrene [P(S-alt-MAH)-b-PS] diblock copolymer, and hexamethylenetetramine, which were cured at elevated temperature. The P(S-alt-MAH)-b-PS diblock copolymer was synthesized via a one-pot RAFT polymerization approach. The morphologies of the nanostructured thermosets were modulated by changing the content of the diblock copolymer. The as-obtained nanostructured phenolic thermosets were further used as the precursors to obtain the mesoporous carbons via pyrolysis under highly pure nitrogen atmosphere. The mesoporous carbons were successfully obtained with adjustable porosity. The mesoporous carbons had specific surface areas of 398.9–491.4 m² g^–1 and pore volumes of 0.22–0.31 cm³ g^–1.