The traditional public-key infrastructure protecting global smart city ecosystems faces an existential danger from the development of quantum computers that are relevant to cryptography. In order to analyze the many obstacles to the adoption of post-quantum cryptography (PQC), this study uses a rigorous sequential explanatory mixed-methods methodology that combines expert interviews (n = 30), a stakeholder survey (n = 300), and computational benchmarking. According to the research, the only method that is presently feasible for near-term deployment is lattice-based Kyber-768, which has a latency of 18.7 ms on edge hardware but has a 2.4 × memory overhead that calls for deliberate hardware refresh cycles. Beyond technical limitations, the analysis reveals a significant socio-technical disconnect: training access (β = 0.67, p < 0.001) is a stronger adoption predictor than financial investment (β = 0.52), and a logarithmic knowledge distribution (R2 = 0.89) produces a 4.5-fold awareness gap between large enterprises (68%) and small-to-medium enterprises (15%). The study forecasts a worldwide transition cost of $2.4 billion with 5–7-year ROI timescales for key infrastructure, and it offers the innovative Policy-Technical Alignment Matrix, a paradigm that maps quantum risks to governance preparedness levels. These findings form the basis of the Quantum Resilience Triad theory, which maintains that concurrent enhancements in algorithmic performance, organizational capabilities, and policy flexibility are required for an effective migration. By providing city authorities with experimentally demonstrated strategies to transform quantum resilience from an abstract problem into a controllable strategic goal, the study creates a new paradigm for transdisciplinary urban cybersecurity research.