It is shown that self-assembled hierarchical porosity in organic polymers can be obtained in a facile manner based on pyrolyzed block-copolymer–phenolic resin nanocomposites and that a given starting composition can be post-modified in a wide range from monomodal mesoporous materials to hierarchical micro-mesoporous materials with a high density of pores and large surface area per volume unit (up to 500–600 m2 g–1). For that purpose, self-assembled cured composites are used where phenolic resin is templated by a diblock copolymer poly(4-vinylpyridine)-block-polystyrene (P4VP-b-PS). Mild pyrolysis conditions lead only to monomodal mesoscale porosity, as essentially only the PS block is removed (length scale of tens of nanometers), whereas during more severe conditions under prolonged isothermal pyrolysis at 420 °C the P4VP chains within the phenolic matrix are also removed, leading to additional microporosity (sub-nanometer length scale). The porosity is analyzed using transmission electron microscopy (TEM), small-angle X-ray scattering, electron microscopy tomography (3D-TEM), positron annihilation lifetime spectroscopy (PALS), and surface-area Brunauer–Emmett–Teller (BET) measurements. Furthermore, the relative amount of micro- and mesopores can be tuned in situ by post modification. As controlled pyrolysis leaves phenolic hydroxyl groups at the pore walls and the thermoset resin-based materials can be easily molded into a desired shape, it is expected that such materials could be useful for sensors, separation materials, filters, and templates for catalysis.