In this study, an experimental investigation is conducted to determine the behavior of RC shear walls found in old and existing buildings that do not comply with the design rules in modern earthquake standards. Scaled reinforced concrete shear wall specimens are built with smooth bars and low concrete quality. The dimensions of the shear wall specimens were selected with an aspect ratio bigger than two as 2500, 1050, and 150 mm for the height, length, and thickness, respectively. Four specimens are representative of nonconforming shear walls, and one wall used as a reference specimen which was designed in accordance with recent building codes using deformed bars. The behavior of the shear walls is determined experimentally by displacement-lateral load relationship under lateral cyclic loading. The study used measurable parameters to investigate the behavior of the test specimens in terms of lateral force capacity, rigidity, ductility, dissipated energy, and displacement components contribution to the total lateral response of the walls. The results showed a substantial loss of stiffness, ductility and energy dissipation capabilities for the tested nonconforming shear walls. Moreover, it is proven in this study that these specimens are governed by the bar slip phenomena which demonstrated more than 80% contribution to the total lateral displacement capacity. In contrast, the reference shear wall exhibited a notable flexural behavior and plastic hinge formation. Additionally, the shear walls built with smooth reinforcement bars lost about 44% of their theoretical potential flexural capacity due to the observed bar slip failure.