Atrial cardiomyopathy is a progressive condition that promotes atrial fibrillation (AF) persistence and adverse outcomes. Although fibrosis has long been considered its hallmark, the histopathological basis in non-valvular AF remains incompletely defined due to limited tissue availability outside surgery or autopsy. To overcome this, we established an integrated approach combining intracardiac echocardiography-guided right atrial biopsy with high-density electroanatomic mapping in AF ablation patients. Our studies revealed that atrial voltage reduction reflects not only fibrosis but also intercellular space expansion, myofibrillar loss, reduced nuclear density, and compensatory cardiomyocyte hypertrophy. Notably, atrial biopsy detected atrial amyloidosis in ~7% of patients, often at an early stage, highlighting its potential for earlier diagnosis and intervention. Sex-based analysis revealed that women consistently had lower atrial voltage, attributable to smaller myocardial mass rather than more severe remodeling. These findings demonstrate that atrial biopsy enables patient-specific histopathological assessment, deepening mechanistic understanding of atrial cardiomyopathy and informing future strategies for AF management.