In recent years, retrofitting of steel structures has emerged as a major issue in structural engineering and construction management. As the adhesive bond is often the critical aspect controlling the actual performance of steel profiles externally strengthened by composite plates, this paper investigates the bond behavior of fiber reinforced polymer (FRP) composites glued to steel substrate; the effect of prestress, being relevant in practical applications, is also covered herein. A simplified mechanical model, based on assuming mode II fracture for FRP-to-steel debonding is formulated at first, and then, a numerical solution is implemented. The proposed solution is validated by experimental tests. Finally, a parametric study highlights the role of several relevant parameters on the response of carbon FRP plates under prestress-release tests; it paves the way for further developments of the present study aimed to predict the prestress force release and lap-shear behavior of FRP-to-steel bonded joints based on the mechanical properties of the materials and the bond–slip relationship assumed for the adhesive interface.