Background: Depression is a known stroke risk factor, but its influence on stroke course depending on gender and age is not clearly defined. The purpose of this study was to determine the impact of previously diagnosed depression on early and one-year mortality in patients with acute ischemic stroke (IS) in relation to gender, age, and concomitant diseases. Methods: This study was based on the registry created from the public health insurer in Poland (2009–2020). Two groups were distinguished: IS-D—patients with IS and a diagnosis of depression within preceding 5 years (n = 520); IS-nD—patients with IS who had never been diagnosed with depression (n = 11,505). Results: In-hospital, 30-day, and 1-year mortality did not differ between groups (3.8% vs. 5.1%, 13.1% vs. 12.9%, and 27.1% vs. 26.8%, respectively). However, when statistical analysis was performed stratified by gender and age, we found a significantly higher 30-day and 1-year mortality in men under the age of 65 with previously diagnosed depression in comparison to those without depression (13.6% vs. 3.8%, p < 0.001; and 20.3% vs. 10.8, p < 0.021, respectively). Conclusions: The incidence of post-stroke depression was higher in IS-D group, regardless of gender and age. Early and one-year mortality was higher in younger men with IS and previously diagnosed depression.