This study investigates seismic retrofitting of the National Theatre building in Prishtina, originally constructed in 1948 and later modified through interventions that altered its structural concept. Nonlinear time-history analyses in TREMURI were used to quantify seismic demand and identify the masonry walls governing global stability. A strengthening scheme based on fiber-reinforced polymer (FRP) and glass-fiber-reinforced polymer (GFRP) mesh embedded in compatible mortars was then evaluated with the same model. For a representative critical wall, the retrofit increased in-plane shear capacity from 467 kN to 682 kN (≈46%), while axial and flexural capacities remained essentially unchanged, and building-level lateral stiffness increased by approximately 10–15%, reducing displacement demand and stress concentrations. The results indicate that lightweight composite reinforcement, applied with minimal invasiveness, can provide measurable seismic performance gains while supporting conservation objectives for heritage theatre buildings.