Adhesive-bonded applications are widely used in industry, because of significant advantages such as uniform stress distribution, design flexibility and suitability to bond similar and dissimilar structural materials. This study focuses the adhesive-bonded long overlap of steel/carbon fibre reinforced polymer (CFRP) composite double lap shear (DLS) joints. The purpose of the work is to predict and assess the structural failure and behaviour of the DLS joint, including delamination of the composite, and to determine the effects of the design parameters of adherend thickness, overlap length and fabric orientation on the joint’s failure. There are different ways for such a joint to fail, which makes predicting failure very difficult. Another important difference is the failure mode of composites, where the relatively low interlaminar shear or tensile strength of the resin system causes failure of the composite before failure of the adhesive bondline occurs. Both experimental and numerical methods were used for the analysis. The experimental programme includes fabrication, mechanical testing and failure examinations of various joints. The numerical methods are based on 2D models, using strength of materials and cohesive zone modelling (CZM) approaches. In order to model adhesive joints accurately and efficiently, fracture tests were implemented to determine the fracture criteria. Mode-I and mode-II fracture energies were obtained by double cantilever beam (DCB) and end notched flexure (ENF) tests. An inverse method was used to define the cohesive parameters of the bilinear relation, fitting the numerical and experimental load-displacement curves. The DLS model has been created in Abaqus software, and results for each approach have been presented. Critical locations of stress concentrations in the DLS joint were identified, and the CZM successfully predicted the delamination initiation and propagation region observed in the experiment. As a result, it was concluded that the data obtained from the analysis showed good agreement with the experimental results, and in addition to the fibre orientation angles of the CFRP laminate markedly affecting the failure load of joints, the failure mode and stress distributions appeared in adhesive and composite. Furthermore, the study shows that the cohesive elements enable the numerical results to be obtained in shorter simulation times than the strength of materials approach, which should encourage use of CZM to analyse large structural applications.