This paper investigates the processing-induced strains developed in the adhesive layer of a multi-material single lap joint. High-resolution distributed optical fiber sensors were embedded in the adhesive for in-situ measurement of axial strains and temperatures, developed during the lap joint manufacturing and cooling process. The single lap joints made of Glass-Fiber Reinforced Plastic (GFRP) and Aluminum Alloy 6061-T6 adherends were bonded using Acrylonitrile Butadiene Styrene (ABS). The coefficient of thermal expansion (CTE) mismatch between the thermoplastic adhesive and optical fiber results in varying interfacial shrinkage, leading to interfacial strain incompatibility. Therefore, the experimental strains recorded by the optical fiber do not represent the actual strains experienced by the adhesive. To overcome that challenge, a detailed thermo-mechanical finite element (FE) model is required to accurately capture the optical fiber’s response within the ABS adhesive bondline. To achieve the objective, the project was divided into two parts: a) an experimental investigation into the strain response of the embedded optical fiber and b) numerical validation. The current work focuses purely on the experimental investigation of the strains induced in the optical sensing fiber during processing. Bonded joints embedded with optical fiber were subjected to two cooling cycles. In a previous study, the measured strains were compared with those obtained for the same-adherend (GFRP) single lap joints. The measured strains were also used to make an analytical estimation of residual strains in the adhesive bondline. A significant increase in experimental strains and analytically calculated residual strains was observed for the multi-material bonded lap joints.