Out-of-pocket expenditure constitutes approximately 70 percent of total health spending in Nigeria, exposing millions of households to catastrophic health expenditure and undermining sustainable healthcare delivery at the facility level. The National Health Insurance Scheme (NHIS), operational since 2005, was established to address this through a pre-payment, risk-pooling mechanism; however, coverage remains at approximately 5 percent of the population, reflecting persistent implementation gaps. This study examined the relationship between the National Health Insurance Scheme and sustainable healthcare delivery at the Nigeria Military Hospital, Port Harcourt, 2015-2025, focusing on the level of enrolment, funding adequacy for sustainability, and the availability of medical consumables. Anchored on Health Insurance Theory and Social Policy Theory, the study was guided by three objectives, three research questions, and three null hypotheses. A mixed-method correlational survey design was adopted from a population of 15,035 persons; 390 respondents were determined using Taro Yamane’s formula through proportional stratified sampling, and 366 questionnaires were retrieved and usable (93.8% response rate), complemented by interviews with 15 key informants. Cronbach’s Alpha reliability coefficients ranged from 0.807 to 0.821. Data were analysed using descriptive statistics and Pearson Product Moment Correlation Coefficient (PPMC) at the 0.05 significance level; qualitative data were subjected to thematic content analysis. Findings revealed statistically significant positive relationships between sustainable healthcare delivery and the level of enrolment (r = 0.614, p < 0.05), funding adequacy (r = 0.579, p < 0.05), and availability of medical consumables (r = 0.598, p < 0.05). All three null hypotheses were rejected. The study recommended that the National Health Insurance Authority enforce monitored HMO reimbursement timelines, that the Federal Ministry of Defence ring-fence supplementary capital allocations for military health facilities, and that a structured pharmaceutical supply management system be institutionalised at the Nigeria Military Hospital, Port Harcourt.