A standard test is needed to characterise the local bond behaviour of carbon fibre-reinforced polymers (CFRP) externally bonded (EB) to a structural material. In the literature, different tests can be found, e.g., single-lap shear or double-lap shear tests with a pull-pull or pull-push configuration or the 4-point bending test. Still, none of these are considered nor recommended as the best test to define the local bond behaviour of CFRP-to-substrate joints. The 4-point bending test is probably the less popular bonding test due to the influence of shear and bending of the beam on the local bond behaviour between the CFRP and the soffit of the beam. Therefore, the effect of fracture mode I on the fracture mode II debonding process of the CFRP composite is still poorly understood. For this reason, a numerical model was developed in which the debonding process of the CFRP-to-substrate under mixed mode is studied (I + II). An experimental program with three hinged concrete beams EB with CFRP composites was carried out to validate the numerical model. Moreover, the Finite Element Method (FEM) is used to model the experimental tests. The proposed numerical model is validated by comparing the load-deflection curves, CFRP strains, and bond stress distributions obtained in each case. The influence of mode I on the mode II debonding process of the CFRP-to-concrete joints is also analysed.