The main purpose of the current study is to investigate bond behavior of fiber reinforced polymer (FRP)-to-concrete under mixed-mode I/II loading. To attain this end, 150 × 150 × 330 mm concrete prisms were cast and strengthened by carbon fiber reinforced polymer (CFRP) sheets having constant widths of 50 mm and varied lengths of 100 and 150 mm in the form of externally bonded reinforcement (EBR) method. Totally, 31 single lap-shear tests were implemented, 4 were control tests conducted in pure mode II loading, and 27 were conducted in a way that the FRP sheet is loaded at an angle ranging from +4.7 to −6.0 degrees with respect to the substrate in order to experience mixed-mode I/II loading. Using a non-contact digital image correlation system, deformation measurements and consequently the load-slip responses of the joints were obtained. The interactions of axial force vs. the out-of-plane normal force, developed in the unit width of FRP at the failure, resulted from mode II and mode I components of loading were also derived. Experimental results confirmed that when the interface experiences mode I component of loading in the form of normal out-of-plane tensile or compressive stresses, the bond strength would respectively decrease or increase over the control specimen. Based on these experimental results, a simple model is also proposed to predict FRP-to-concrete bond strength under mixed-mode loading.