Background/Objectives: Previously undiagnosed atrial fibrillation (PUAF) is a significant cause of embolic stroke of undetermined source (ESUS). This study aimed to determine whether early heart rhythm monitoring with a Holter ECG after acute stroke enhances the detection of PUAF compared to standard ambulatory monitoring in ESUS patients, assuming that early cardiac monitoring would lead to a higher detection rate of PUAF. Methods: This cohort study included 100 patients aged 50 and older diagnosed with ESUS and exhibiting sinus rhythm for at least 24 h. All participants were hospitalized in a stroke unit and underwent 48 h of Holter ECG monitoring. A group of 100 ESUS patients who underwent outpatient delayed Holter ECG monitoring served as controls. Results: This study revealed a significantly higher detection rate of AF in the hospitalized group compared to the outpatient group (20% vs. 5%; p = 0.001). The mean age and distribution of risk factors, including hypertension, diabetes, hyperlipidemia, ischemic heart disease, heart failure, chronic kidney disease, smoking, previous stroke, and malignancy, did not differ between the groups. There were no significant differences in initial stroke severity or in outcomes between the groups. Conclusions: Early Holter ECG monitoring in the hospitalized ESUS patients significantly increased the detection rate of PUAF compared to ambulatory monitoring, highlighting the importance of timely cardiac assessment in stroke management.