Time-dependent mechanochemical activation of spiropyran (SP) mechanophores in cross-linked PMMA under creep loading is examined. In contrast to monotonic torsion (constant strain rate) results, the desired force-induced electrocyclic ring-opening of SP to the fluorescent merocyanine (MC) form is achieved at creep stress levels less than the yield stress of the bulk polymer. Lower values of stress require longer time, but much smaller strains for initiation of activation. Additionally, the first measurable detection of the reaction corresponds closely to the maximum creep strain rate, revealing mechanophore activation occurs near the onset of strain hardening, when polymer mobility is highest. The strong correlation between mechanical activation of the spiropyran and the rate of deformation suggests the potential for SP mechanophores as molecular probes of polymer relaxation and flow.