Aims <p>To compare the 1-year prognoses of patients with atrial fibrillation known before stroke (KAF) with those diagnosed after (AFDAS) and to explore the reasons for any observed differences.</p> Methods <p>420 ischemic stroke patients were assigned to the KAF and AFDAS group. Follow-up information for both groups included the incidence of ischemic stroke recurrence, poor neurofunctional outcomes, and all-cause mortality within one year after the original ischemic stroke episode. The differences in the two groups' prognoses were assessed, and Cox/logistic regression models were employed to identify the underlying factors influencing this prognostic gap.</p> Results <p>AFDAS had significantly lower rates of ischemic stroke recurrence (11.06% vs. 19.34%, <i>P</i> = 0.018) and poor neurofunctional outcome (25.48% vs. 36.79%, <i>P</i> = 0.012) compared to KAF. One-year all-cause mortality was similar between the two groups (7.69% vs. 9.91%, <i>P</i> = 0.424). AFDAS had a significantly lower risk of ischemic stroke recurrence even after adjusting for mortality factors (HR, 0.542; 95% CI, 0.325–0.903; <i>P</i> = 0.019). The incidence rate of ischemic stroke recurrence was significantly lower in AFDAS compared to KAF after adjustment for non-cardiac factors (HR, 0.561; 95% CI, 0.316–0.995; <i>P</i> = 0.042). However, there was no discernible difference in the two groups after adjustment for cardiac factors (HR, 0.659/0.588; 95% CI, 0.390–1.115/0.327–1.058; <i>P</i> = 0.120/0.077). The probability of a poor neurofunctional outcome was 3.758-fold higher in patients with recurrent ischemic stroke compared to those without recurrence (HR, 3.758; 95% CI, 1.587–8.900; <i>P</i> = 0.003). Compared to KAF, AFDAS was 88% less likely to have a poor neurofunctional outcome (HR, 0.120; 95% CI, 0.052–0.277; <i>P</i> &lt; 0.001).</p> Conclusion <p>Patients with AFDAS have a better prognosis than those with KAF.</p>