<p>Soft brick masonry infills are widely used in reinforced concrete (RC) frames in developing countries, particularly in India. However, most existing experimental and numerical studies have focused on stiff brick masonry, and there is limited literature specifically addressing the behaviour of RC frames with soft brick unreinforced masonry (URM) infills. The frame–infill interaction is governed primarily by their relative stiffness, meaning that conclusions drawn from stiff infill research cannot be directly extrapolated to soft brick infills. This study investigates the in-plane behaviour of RC frames with soft brick masonry infills through detailed numerical analysis. A micro-modelling approach is adopted to accurately represent the soft brick URM infill. Monotonic lateral loading is applied to examine the complex frame–infill interaction mechanisms and to evaluate interfacial tractions. A parametric study is conducted to assess the influence of infill wall aspect ratio on lateral strength, stiffness, and failure modes. The development and characteristics of diagonal compression struts are analysed, and effective strut widths are determined. Furthermore, nonlinear time-history analyses (NTHA) are performed on the infilled frame using the equivalent strut modelling approach. The results are combined with limit states prescribed by relevant Indian Standards (IS codes) as well as additional damage states identified from the detailed micro-models. Seismic fragility curves are derived for the bare frame, the infill panel alone, and the composite infilled frame. The fragility assessment reveals that infilled frames with higher aspect ratios exhibit significantly lower seismic vulnerability, sustaining considerably higher peak ground acceleration (PGA) levels before reaching various damage states.</p>