This study evaluates the sensitivity of modal parameters to incomplete adhesion in single-lap adhesive joints by comparing the vibrational behavior of fully bonded joints and joints with a macroscopic partial bond (≈50% loss of bonded area in the overlap). Beyond prior studies on crack-like or delamination-type defects, often using intrusive or destructive inspections, the present work targets a practically relevant macroscopic manufacturing flaw within a fully non-destructive modal testing framework. An integrated experimental–numerical approach is employed: aluminum 7075-T6 beams bonded with epoxy 828 are tested using impact-hammer excitation and laser Doppler vibrometer, and responses are processed in MATLAB and MEscope to extract natural frequencies and damping ratios. A three-dimensional finite element model in Abaqus/CAE is refined via limited experimental calibration. Results show 3–15% reductions in the first eight bending natural frequencies of defective joints relative to intact ones, with stronger effects in higher modes due to dominant stiffness loss over minor mass reduction. The damping ratio increases about sevenfold (0.0032 to 0.0236) in the defective specimen, attributed to frictional dissipation at the unbonded interface and contact nonlinearities supported by mode-shape analysis. These quantified shifts in frequency and damping establish modal parameters as reliable, non-destructive indicators of incomplete adhesion in single-lap joints and provide a basis for vibration-based structural health monitoring and defect detection in adhesively bonded components in aerospace, automotive, and multi-material structures.