Out-of-plane failure of masonry infill walls is a critical issue affecting the seismic performance of reinforced concrete (RC) frame structures, especially during strong ground motions. While infill walls enhance the in-plane stiffness of RC frames, they remain highly vulnerable to out-of-plane actions perpendicular to their surface. This study presents a detailed numerical investigation of the out-of-plane behaviour of masonry infill walls with various boundary conditions using the finite element software ABAQUS. Four support configurations were considered: infills supported on four sides, three sides, two horizontal sides, and one side. The analyses focused on deformation patterns, stress distribution, and failure mechanisms under out-of-plane loading. The results demonstrate that boundary conditions have a significant influence on wall stiffness, strength, and overall stability. Full boundary confinement (four-sided support) markedly improves performance through the development of a two-way arching mechanism. Quantitatively, the four-sided infills exhibited approximately 73% higher out-of-plane strength than the three-sided configuration, about 34% higher than the two-sided configuration, and substantially greater resistance than the single-sided case. These findings highlight the crucial role of boundary support in the seismic evaluation and retrofitting of infilled RC frame structures.