Adhesive bonding is an attractive joining method for fibre reinforced polymers (FRPs); however, if adhesive joints are to be used in structural applications, methods of accurately predicting mechanical performance are required. In many structural applications the joints will be subjected to cyclical loading, in which case fatigue performance must be considered. One of the most useful methods of analysing fatigue is through the application of fracture mechanics, in which the fatigue crack growth rate is related to an appropriate fracture mechanics parameter, such as strain energy release rate. This relationship is often plotted on a fatigue crack growth (FCG) plot and an empirical FCG law derived by fitting an appropriate form of equation to this plot. However, in bonded composite joints, anomalous FCG is seen when mixed mode samples are tested. In this paper, this anomalous behaviour is explained and a mechanistically based model for predicting this behaviour is proposed. The model is applied to experimental data and a good fit is seen.