Poly(vinyl alcohol)- (PVA-) based activated carbon fibers (ACFs) with a high mesoporosity have been fabricated. The surface area and pore size distribution were fine-tuned by the careful control of the activation temperature and time. A mesopore volume fraction of 66% was obtained by activation at 1173 K for 120 min or 1273 K for 90 min. Raman spectroscopy revealed that more defect sites in the graphitic structure were found in samples with higher mesoporosity. Adsorption experiments demonstrated that the PVA-ACFs exhibited good adsorption capacities for both small-molecular-size iodine and the larger-size methylene blue, with adsorption capacities of up to 1934 and 709 mg/g, respectively. The adsorption kinetics of methylene blue on the PVA-ACFs was simulated by both a pseudo-second-order kinetic model and an intraparticle diffusion model. With an increase in mesoporosity, rate limitations on adsorption due to intraparticle diffusion were dramatically reduced. This resulted in an increase of up to a factor of 7 in the adsorption rate compared to that on microporous PVA-based ACFs.