The radiative properties of interstellar dust are affected not only by the grain size distribution but also by the grain porosity. We develop a model for the evolution of size-dependent grain porosity and grain size distribution over the entire history of galaxy evolution. We include stellar dust production, supernova dust destruction, shattering, coagulation, and accretion. Coagulation is assumed to be the source of grain porosity. We use a one-zone model with a constant dense gas fraction (η_dense), which regulates the balance between shattering and coagulation. We find that porosity develops after small grains are sufficiently created by the interplay between shattering and accretion (at age t ∼ 1 Gyr for star formation time-scale τ_SF = 5 Gyr) and are coagulated. The filling factor drops down to 0.3 at grain radii ∼ 0.03 μm for η_dense = 0.5. The grains are more porous for smaller η_dense because small grains, from which porous coagulated grains form, are more abundant. We also calculate the extinction curves based on the above results. The porosity steepens the extinction curve significantly for silicate, but not much for amorphous carbon. The porosity also increases the collisional cross-sections and produces slightly more large grains through the enhanced coagulation; however, the extinction curve does not necessarily become flatter because of the steepening effect by porosity. We also discuss the implication of our results for the Milky Way extinction curve.