Introduction: The admission blood glucose (ABG) level is associated with increased mortality in non-diabetics patients with Coronavirus disease-2019 (COVID-19) in short-term follow-up studies. However, post-discharge mortality has also increased in COVID-19. Thus, this study aimed to examine the relationship between ABG and 90-day mortality including the post-discharge period. Methods: Non-diabetic patients who are hospitalized due to COVID-19 in 2020 were evaluated. Patients were divided into groups according to the ABG level. Groups 1, 2, and 3 have ABG level of <100 mg/dL, 100-139 mg/dL, and 140-199 mg/dL, respectively. Intensive care unit admission, in-hospital mortality, and 30- and 90-day mortality rates were evaluated as outcomes. COX regression analyzes were used to assess mortality risk factors. Results: A total of 1207 non-diabetic patients, of whom 49.2% were females, with a mean age of 65.2±13.4 years, were included in the study. The patients were followed up for a median of 153 (inter quartile range: 107.5-251, maximum: 369) days. The in-hospital and 30-day mortality of group 2 was higher than group 1 in the univariate analysis but without statistical significance in multivariate analysis. Group 3 had worse outcomes than group 1 in both univariate and multivariate analysis at all endpoints. Group 3 had 2.533 adjusted hazard ratios (95% confidence interval: 1.628-3.941, p<0.001) 90-day mortality compared with group 1. Conclusion: Non-diabetic patients with COVID-19 with an admission glucose level of ≥140 mg/dl had 2.5-fold increased all-cause mortality at 90 days. Therefore, being more careful in treating and following non-diabetic patients with COVID-19, especially those with hyperglycemia at admission, was recommended.