Left atrial (LA) strain is clinically used to assess LA function, yet its relationship to underlying mechanics and fibrosis remains incompletely understood. We used a physiologically detailed 3D LA electromechanical–0D circulatory model to evaluate fibrosis‑induced changes, combined with patient‑specific geometries from atrial fibrillation (AF) and embolic stroke of undetermined source (ESUS) cohorts. Reservoir (total), booster (active), and conduit (passive) function were quantified using volumetric indices (emptying fraction, EF), 3D fiber‑direction strain, and 2D cross‑sectional strain. Booster function showed the largest fibrotic reductions, with highest relative differences in 2D strain (47% reduction from baseline) and strongest correlation with fibrosis burden in 3D strain (R^2 = 0.84). Our findings show that computational models reproduce fibrotic effects within clinically representative ranges and may help bridge computational modeling and clinical assessment.