(1) Background: Early-onset atrial fibrillation (AF) exhibits distinct clinical and genetic profiles compared to AF in older adults. The increasing detection of AF among younger patients—often in the absence of traditional risk factors—has raised interest in the genetic determinants underlying the condition; this review aims to synthesize current evidence on the genetic architecture of early-onset AF, assess the clinical utility of genetic testing, and discuss future directions for integrating genetic insights into personalized management strategies (2) Methods: We conducted a comprehensive analysis of recent studies, including genome-wide association studies and targeted sequencing efforts, that examined rare pathogenic variants and polygenic risk scores in early-onset AF. The review also considers emerging data on atrial cardiomyopathy and evaluates current guideline recommendations for genetic testing; (3) Results: Data indicate that rare variants, particularly in genes such as TTN, LMNA, and KCNQ1, play a significant role in early-onset AF, with evidence suggesting an association between these mutations and adverse clinical outcomes. Polygenic risk scores further complement traditional risk factors, providing a more nuanced risk stratification. Despite these advances, challenges remain in the interpretation of variants of uncertain significance, cost-effectiveness, and the need for interdisciplinary collaboration in clinical implementation; (4) Conclusions: Integrating genetic evaluation into the diagnostic and management framework of early-onset AF holds promise for improved risk stratification and personalized therapy. Future large-scale, multi-ethnic studies and ongoing refinement of genetic risk models are essential to overcome current limitations and enhance the clinical applicability of genetic testing in this rapidly evolving field.