Urban densification compels architects to extend structures vertically, yet increased slenderness amplifies vulnerability to dynamic excitation. The generated vibrations exceed occupant-comfort thresholds and accelerate structural fatigue. The dynamic response of slender high-rise structures to wind and seismic loads remains a significant challenge in sustainable building design. This paper proposes an adaptive pendulum-tuned mass damper (APTMD) equipped with an adjustable-length cable that enables real-time modal retuning, for example, through controlled fluid transfer. A coupled APTMD-based system model was formulated using Lagrange equations, linearised descriptors for small perturbations were derived, and parametric studies were performed using a proprietary Python framework (DynPy). Next, numerical investigations of system dynamics based on APTMD were performed. Furthermore, experimental validation was conducted by APTMD performance tests on a vibration table using a reduced-scale building frame model. Numerical simulations indicate that adaptive cable adjustment suppresses resonance amplitudes by up to 85% relative to a fixed-length TMD. In comparison, experimental trials on a scaled prototype confirm a comparable 80% reduction in peak acceleration. The findings establish the proposed variable-length APTMD as a low-energy, retrofit-friendly strategy for enhancing the resilience and sustainability of next-generation skyscrapers. This points to a viable pathway toward energy-efficient, occupant-friendly high-rise buildings. Additionally, it may contribute to improving the seismic safety of buildings. The proposed approach demonstrates strong potential for implementation in modern engineering structures that require active vibration control under variable dynamic loads, such as wind or seismic activity, as well as stiffness changes caused by exploitation.