In fiber-reinforced composites, the interphase nanostructure (i.e., the extended region between two phases in contact) has a pronounced influence on their interfacial adhesion. This work aims at establishing a link between the interphase design of PS-based polymeric fiber coatings and their influence on the micromechanical performance of epoxy-based composite materials. Thiol–ene photochemistry was utilized to introduce a polymeric gradient on silica-like surfaces following a two-step approach without additional photoinitiator. Two complementary grafting-techniques were adapted to modify glass fibers: “Grafting-onto” deposition of PB-b-PS diblock copolymers for thin-film coatings (thickness <20 nm) at low grafting density (<0.1 chains/nm²) - and “grafting-from” polymerization for brush-like PS homopolymer coatings of higher thickness (up to 225 nm) and higher density. Polymer-coated glass fibers were characterized for polymer content using thermogravimetric analysis (TGA) and their nanostructural morphologies by scanning electron microscopy (SEM). Model substrates of flat glass and silicon were studied by atomic force microscopy (AFM) and spectroscopic ellipsometry (SE). The change in interfacial shear strength (IFSS) due to fiber modification was determined by a single fiber pull-out experiment. Thick coatings (>40 nm) resulted in a 50% decrease in IFSS. Higher shear strength occurred for thinner coatings of homopolymer and for lower grafting densities of copolymer. Increased IFSS (10%) was found upon dilution of the surface chain density by mixing copolymers. We show that the interfacial shear strength can be increased by tailoring of the interphase design, even for systems with inherently poor adhesion. Perspectives of polymeric fiber coatings for tailored matrix–fiber compatibility and interfacial adhesion are discussed.