Background: The COVID-19 pandemic has resulted in significant morbidity and mortality worldwide, prompting the development and distribution of effective vaccines. The impact of vaccination status on COVID-19 patient outcomes remains a critical area of investigation to inform public health strategies and promote vaccine uptake. This study was conducted to investigate the association between COVID-19 vaccination status and mortality in patients admitted to a tertiary care centre in Karnataka. Methods: A retrospective cohort study was conducted involving 300 patients, divided into three groups: 100 completely vaccinated, 100 partially vaccinated, and 100 unvaccinated. All patients were symptomatic for COVID-19 and admitted to HIMS hospital, where SARS-CoV-2 was detected using Real-time reverse transcriptase polymerase chain reaction (RT-PCR) and Rapid Antigen Test (RAT). Mortality outcomes were compared across the three groups, adjusting for age, sex, and comorbidities. Results: The mortality rate was significantly lower in the completely vaccinated group (8%) compared to the partially vaccinated (20%) and unvaccinated groups (32%) (p < 0.05). The adjusted odds ratio (OR) of mortality, after controlling for confounding factors, was 2.42 (95% CI: 1.01-5.81) for the partially vaccinated group and 3.12 (95% CI: 1.25-7.79) for the unvaccinated group compared to the completely vaccinated group. Conclusion: Our findings demonstrate a significant protective effect of complete COVID-19 vaccination against severe outcomes, including mortality, among patients admitted to a tertiary care center in Karnataka. These results emphasize the importance of widespread vaccination campaigns to mitigate the impact of the COVID-19 pandemic and encourage public health efforts to promote vaccine uptake.