The gradual damage behaviour of polar orthotropic glass-fibre reinforced epoxy disc-rotors and the resulting damage-dependent dynamic behaviour is experimentally investigated and described under propagating damage from a combination of out-of-plane and in-plane loads. The damage manifests itself as a spatial increase of inter-fibre failure and growth of delaminations, resulting in local changes of stiffness and damping. A novel observation is reported finding that a monotonic increase of damage results in a significant non-monotonic frequency shift of significant amount at the investigated eigenfrequencies. Furthermore, a simulation tool is applied, which calculates the damage-dependent dynamic behaviour considering both damage initiation and evolution. Within this model, damage initiation is determined by means of stress-based failure criteria. The gradual damage evolution is then described with a validated continuum damage mechanics model, which captures the degradation processes after considering failure modes and damage coefficients. By comparing the experimental and numerical results, the corresponding damage and vibration response behaviour are analysed and evaluated. Based on the experimental results, it can be deduced that a monotonic increase of damage can result in a non-monotonic frequency change, which is also observed in the numerical results from the simulation analysis.