Reinforced concrete (R.C.) buildings are highly vulnerable to seismic failures due to soft story mechanisms and low ductility.Steel-concrete composite frames offer improved ductility, lateral load resistance, and energy absorption under seismic forces. This studyinvestigates the seismic performance of steel-concrete composite buildings with and without masonry infill walls using a probabilisticfragility-based approach. A fifteen-story composite frame is analyzed in bare and infill configurations through non-linear static pushoveranalysis, and fragility curves are developed to assess damage probabilities at various performance levels.The results demonstrate that composite infill frames significantly improve lateral stiffness, base shear capacity, and seismic resiliencecompared to bare frames. Incorporating masonry infill substantially reduces the probability of failure across all damage states. Additionally,seismic performance assessments of low-rise, mid-rise, and high-rise composite buildings using Incremental Dynamic Analysis (IDA)reveal that low-rise structures exhibit reduced dispersion and more predictable seismic responses.This research highlights the effectiveness of masonry infill and composite construction in enhancing seismic safety and provides a valuableframework for performance-based seismic design in earthquake-prone regions.Keywords: Base Shear, Story Displacement, Story Drift, IS 800-2007, Response Spectrum Method