This research addresses the implementation of passive seismic protection through the design and construction of a lead-core elastomeric base isolator for the three-story L-shaped Instituto Inocencio Jácome building in Quito, Ecuador. The structure, composed of reinforced concrete frames, was retrofitted with isolators installed at the ground level to reduce seismic demand. The design process began with a pre-dimensioning stage and continued with a dynamic structural analysis using the modal superposition method under two seismic hazard levels—rare and very rare—according to the NEC-15 elastic spectrum. A planar frame model incorporating the isolators as vertical spring-damper elements was analyzed under multiple load combinations to assess displacements, internal forces, and rotation demands. The isolator's final design was refined through verification of rubber layer thickness, internal steel shims, external plates, and stability against buckling and excessive deformation. To validate the theoretical behavior, an experimental test was performed on a full-scale prototype under cyclic axial and lateral loading. The resulting force-displacement data allowed the generation of hysteresis curves, confirming energy dissipation capacity and nonlinear elastic performance under seismic actions. This study demonstrates the technical viability and performance benefits of lead-core elastomeric isolators for seismic retrofitting of mid-rise buildings in high-risk zones like Quito.