Predictive models have struggled to accurately simulate progressive delamination growth in composite structures, often due to the challenges associated with modelling delamination migration phenomenon. This paper presents a methodology with which to model such migration. Firstly, the interlaminar shear at a delamination front were partitioned into axial and transverse modes, the mode-mixity of which was controlled by the mismatch between this front and the ply directions. An element formulation was presented which utilised this mismatch. Consequently, delamination migration was shown to have ensued when the transverse mode exceeded a critical mode-mixity Giii/GT = 0.22. This approach was verified against experimental studies on width tapered end loaded split coupons by correlating the fracture morphology against mode-mixity. In particular, the orientation of the cusps on the delamination surface were shown to be controlled by the relative dominance of the axial and transverse modes. This methodology provides a means to accurately and robustly model progressive delamination growth processes in composite structures.