The effect of the supporting conditions on the overall and the localized out-of-plane behavior of un-reinforced masonry (URM) walls strengthened with externally bonded composite materials is analytically investigated. Four combinations of supporting conditions with different restrictions of the longitudinal elongation and rotation are examined. These cases reflect some of the possible realistic supporting conditions that may exist in practical applications. A theoretical model that is based on variational principles, equilibrium equations, and compatibility requirements between the structural components (masonry, mortar, FRP, adhesive) is used for the investigation. The effect of cracking of the mortar joints and the interaction of the existing wall with the composite strengthening system through the adhesive layer are considered. The results reveal the critical influence of the various supporting conditions on the response of the strengthened wall and shed some light on aspects of the design of the externally bonded strengthening system.