Background and Purpose Atrial fibrillation (AF) is a leading risk factor for ischaemic stroke (IS). Worldwide, population-based data on its influence on IS outcomes are sparse. In New Zealand (NZ), there have been no previous population-based incidence studies of IS in patients with AF. The study's first aim was to measure the prevalence of AF in patients with first-ever IS. The secondary purpose was to compare the level of adherence with oral anticoagulant (OAC) therapy (mainly warfarin) among IS patients with AF before and after the stroke event. Thirdly, I sought to evaluate the impact of AF on outcomes during a 12 month follow-up after the IS event. Methods I evaluated the prevalence of AF and its influence on prognosis in patients with a first-ever IS. The study forms an extension to the fourth Auckland Regional Community Stroke Study (ARCOS IV) conducted in 2011-2012, a population-based stroke incidence and outcomes study undertaken to identify stroke cases in the resident population of Auckland, New Zealand aged 15 years or older. Stroke cases were ascertained from multiple overlapping sources. AF was defined as new (post-stroke) or pre-existing (pre-stroke). In the late stages of this study, direct OACs were starting to be introduced into practice. The adherence with warfarin therapy was assessed using INR levels and TTR (time in therapeutic range) values. Logistic regression analysis was run to determine associations between AF, clinical factors, and demographic variables in patients with IS. Cox regression and Kaplan Meier analyses were run to estimate and model patients’ survival up to 12 months following the stroke event. Results A standard electrocardiogram confirmed AF at stroke onset and during the acute phase in 421 of 1,329 (31.6%) patients with IS. The prevalence of AF among IS patients is similar to that found in other developed countries. The mean age of IS patients with AF was 77.9 ± 11.7 years. Those with AF were more frequently females aged 75 years and over. Regression analysis to predict first-ever IS in patients with AF found that sex (p = 0.002), ethnicity (p < 0.001) and age (p = 0.044) add significantly to the model prediction, whereas diabetes (p = 0.466), HTN (p = 0.723), vascular disease (p = 0.592) and TIA (p = 0.185) do not. The presence of AF was associated with higher case fatality and stroke recurrence rates. There was a positive association between death at 12 months and the risk of thromboembolism, MI post-stroke, treatment, age, ethnicity, TIA post-stroke and stroke severity. I found that the hazard of dying within 12 months post-stroke is lower for patients on warfarin. Of 92 patients on warfarin before stroke for whom TTR was calculated, only 36 (35.3%) had a good level of adherence. Conclusions There is a high prevalence of AF in older patients with first-ever IS. In addition, AF is strongly associated with a high risk of death after a first-ever IS. Better control of vascular risk factors combined with broader use of OACs may reduce the risk of recurrent stroke, myocardial infarction (MI), and death in patients with IS and AF.