The seismic resistances of RC buildings with masonry infill (MI) walls confined by Reinforced concrete (RC) frames are affected by the crucial role, either positive or negative, played by these walls. In this study, a nonlinear numerical investigation on the lateral behavior of masonry infilled RC buildings is carried out. Variety of parameters for both MI walls and buildings are considered. Different configurations of MI walls, size of wall openings, absence of MI walls in the first storey and MI wall thickness are investigated. The application buildings are either moment resisting frames (MRF) or dual shear wall-moment resisting frames (SW-MRF) buildings. The MRF buildings have 6 floors, while the SW-MRF buildings have 5 different heights represented by the number of floors (from six to twenty floors). Equivalent strut methodology is used and modified to model the behavior of infill walls taking into consideration the effect of opening sizes. Nonlinear static push-over analysis is carried out for the applied case study buildings. It is found that MI walls can highly increase the base shear capacity of either building types while significantly reduce the displacement capacity of MRF buildings, RC shear walls can resist this negative effect. The existence of soft first storey can drastically alter the lateral response of buildings. The influence of MI walls fades as the building height increases. The new distribution of failure mechanism is introduced.