Outdoor Trespass Lighting (OTL) is regulated globally using vertical illuminance limits; however, significant divergence exists across six primary regulatory frameworks that were analysed (IES, CIE, ILP, AS/NZS, IDA, TAC), resulting in regulatory fragmentation and inconsistent environmental protection. This study develops and validates a unified international OTL framework using a minimum-threshold convergence (MTC) methodology, whereby the most restrictive permissible illuminance across standards is adopted for each environmental zone. A systematic comparative review (2010–2025) was conducted using Scopus, Web of Science, and professional standards databases. Regulatory divergence was quantified, and a harmonised four-zone framework (1, 3, 8, 15 lux) was derived. The framework was validated through simulation-based case studies using representative roadway lighting geometries and cross-comparison against IES RP-8 and CIE 150 compliance outcomes. Results demonstrate that the unified thresholds: (i) remain within all major international limits, (ii) reduce regulatory ambiguity, and (iii) do not compromise safety or compliance. Sensitivity analysis confirms that minimum-threshold convergence provides a robust, conservative, and legally defensible harmonisation strategy. The study establishes a scalable model for global lighting governance, identifies the limitations of facade-based photometry, namely that vertical illumination does not fully capture circadian or biological light exposure, and recommends integrating it with eye-level and circadian metrics. This is a governance consequence for municipalities and engineering practitioners.