This study examined the influence of regulatory frameworks and environmental sustainability on strategic asset management of high-voltage transmission lines and substations at the Kenya Electricity Transmission Company Limited (KETRACO). The study was drawn from a broader thesis on the determinants of strategic asset management and focused on two governance- and sustainability-oriented variables. A mixed-methods design was adopted, targeting all 180 Operations and Maintenance personnel involved in high-voltage transmission asset management through a census approach. Quantitative data were collected using structured electronic questionnaires, while qualitative insights were obtained through focus group discussions. A total of 128 usable questionnaire responses were obtained, representing a 71.1% response rate. Descriptive statistics, Pearson correlation and multiple regression analyses were conducted using SPSS Version 23, while qualitative data were analysed thematically. Regulatory engagement recorded the highest mean (M = 3.86), followed by regular audits and physical inspections (M = 3.80), whereas licensing and permit acquisition recorded the lowest mean (M = 3.58). For environmental sustainability, environmental assessment and planning recorded the highest mean (M = 3.75), followed by vegetation and ecosystem management (M = 3.69), while waste-to-energy adoption recorded the lowest mean (M = 3.23). Regulatory frameworks were positively associated with strategic asset management performance (r = .603, p < .01), while environmental sustainability also showed a positive relationship (r = .567, p < .01). In the broader study's four-predictor regression model, regulatory frameworks (β = .181, p = .005) and environmental sustainability (β = .148, p = .017) both had statistically significant positive effects. The study concludes that effective strategic asset management requires not only technical and financial capability but also predictable regulatory processes, robust compliance systems, sound environmental planning and practical sustainability measures that enhance the resilience and legitimacy of transmission infrastructure.