Adhesively bonded composite joints find extensive use in aerospace and wind turbine blades subjected to cyclic loads during their lifetime. Bondline defects during manufacturing are regions of fatigue damage onset and progression. The current study evaluates the stiffness degradation of uni-directional glass fiber-reinforced polymer (UD-GFRP) composite adhesive joints with embedded ‘artificial’ defects via constant amplitude fatigue tests. A multi-instrumental setup comprising an extensometer for volumetric stiffness degradation assessment, two-dimensional digital image correlation for full-field displacement measurements, and infrared thermography measuring the full-field surface temperatures developed due to inelastic and irreversible dissipative mechanisms associated with cyclic loading have been used to assess the damage incipience and progression. Based on the first and second harmonics of the transient full-field surface temperature data coupled with the volumetric stiffness degradation data, a nonlinear regression model has been developed that establishes a relationship between stiffness degradation, bondline defect size/location, and applied constant amplitude fatigue. Analyzing the recorded thermal signal in the frequency domain helps delineate the thermal parameters associated with thermoelastic and dissipative mechanisms during damage progression. Additionally, the impacts of process defects during the fabrication of composite joints are studied. The model’s effectiveness in evaluating the stiffness degradation correlated with the thermographic parameters in the presence of these introduced defects is sought.