Optical responses of mechanophore (stress-responsive materials) in fiber reinforced polymer composites under mechanical loads were characterized. A new experimental system was developed to capture in situ mechanophore activation by recording ultraviolet (UV) excited fluorescence during uniaxial load tests. Anthracene-based mechanophore, dimeric 9-Anthracene carboxylic acid (Di-AC) was synthesized and incorporated into an epoxy-based thermoset successfully. This Di-AC embedded epoxy mixture was applied to glass fiber fabric to fabricate mechanophore embedded glass fiber reinforced polymer (GFRP) composites through hand-layup process. Quasistatic and cyclic loads were performed to investigate the effect of different types of loads on mechanophore activation. The results indicated that mechanophore activation occurred at the beginning of the test during the quasistatic loading test and continued linearly before yield. Microcracks were formed in the matrix prior to yield, and UV intensity of the mechanophore exhibited nonlinear response. During fatigue tests, the intensity of fluorescence increased after a certain number of cycles. Microcracks were initiated around the middle stage of the fatigue test, the intensity also showed a nonlinear response. The potential of anthracene-based mechanophore for early damage detection in GFRP under complex loading was observed.