Mechanochemistry has proved to be a powerful tool to map the stress distribution and quantify covalent bond scission occurring when model polymer networks, such as elastomers and gels, are deformed and fractured. In the current work, we incorporate a mechanochromic non-scission type mechanophore, spiropyran (SP), in the elastomeric matrix of nanosilica-filled cross-linked poly(ethylacrylate) nanocomposites (PEANC) containing different filler volume fractions and different filler/matrix interfacial properties. The branched fractal-like morphology of the fillers (characterized by X-ray scattering, AFM, and SEM) and the mechanical properties of our samples in uniaxial tension are similar to the industrially used elastomer nanocomposites. Under tensile loading, the PEANC samples change their color and the concentration of mechanophores activated into merocyanine can be quantified from absorption spectra. Results show that, in uniaxial tension, SP activation is governed by the peak nominal stress applied to the sample resulting in a master curve of activated SP fraction as a function of stress, independent of the filler volume fraction and interfacial coupling. Upon several loading cycles to the same stretch level, the concentration of activated SP decreases moderately, especially at high stretches. The activation of mechanochromic molecules supports the hypothesis of a two-stages toughening mechanism of nanocomposites. At low strain, the load is mostly carried by fractal-like aggregates of silica filler, while at high strain, the load is transferred from the physical network of filler particles to the highly stretched polymer chains surrounding the aggregates. This scenario supports the dominant role played by the limited extensibility of polymer chains in the nanocomposite stiffening. The labeling of polymer networks with mechanosensitive molecules provides here a clear visual pathway to the mechanisms responsible for stiffening, and ultimately toughening, of nanocomposites.