Several existing reinforced concrete (RC) structures are unable to fulfil the current structural safety standards with respect to seismic action, owing to their inadequate structural detailing. However, recent Government policies tend to favour the reuse of older buildings, such that the seismic retrofit of reinforced concrete structures results to be an increasingly common request for civil engineers. In this context, structure-level techniques, including the implementation of exterior steel bracing systems known as “exoskeletons”, can help enhance the safety of existing structure (having low lateral stiffness and numerous non-conforming members) without causing any occupancy disruption. Nevertheless, the development of an accurate numerical model, reproducing both the nonlinear response of brace elements, existing RC members and infill panels, is fundamental to evaluate properly the capacity of the retrofitted structure, especially when damage to non-structural components needs to be prevented. Hence, the present paper outlines the performance assessment of an archetype existing RC structure (located in a high-risk site in Italy) equipped with steel exoskeletons, comparing two retrofit scenarios and considering both serviceability and collapse performance levels. To this end, nonlinear static and dynamic analyses were carried out, to highlight the role of masonry infills on the performance of the retrofitted frame structure.