Background: Cardiac amyloidosis (CA) is mainly characterized by diastolic dysfunction, with gradually worsening functional capacity and poor prognosis. Left atrial (LA) strain by speckle tracking echocardiography (STE) is an index of diastolic function and heart failure (HF) symptoms. The aim of this study was to evaluate the relationship of LA strain with functional capacity in CA and the potential prognostic value of speckle tracking variables. Methods: In this single-center study, we prospectively enrolled consecutive outpatients with CA (n = 75). Clinical, echocardiographic evaluation, six-minute-walking-test (6MWT) and Kansas City Cardiomyopathy Questionnaire (KCCQ) were performed on the same day. The primary endpoint was the correlation between global peak atrial longitudinal strain (PALS) and NTproBNP, 6MWT score, and KCCQ. The secondary endpoint was a combination of all-cause or cardiovascular death and HF hospitalization. Results: Overall, 48 ATTR and 27 AL patients (74 ± 11 years, 84% male) were enrolled. Global PALS showed a significant direct correlation with N-terminal-pro-brain natriuretic peptide (NTproBNP, p = 0.3, p = 0.017) and 6MWT (p = 0.4, R² = 0.2, p = 0.004), but no significant correlation with KCCQ (p = −0.13, p = 0.3). GLS showed a significant direct correlation with NTproBNP (p = 0.3, p = 0.017) but not with 6MWT and/or KCCQ. Over a mean follow up of 12 ± 3 months, 42 patients reached the combined endpoint. With ROC curves, both global PALS < 13.5% and GLS > −12% provided a good prediction of the combined endpoint (AUC = 0.72 [0.6–0.82] and 0.73 [0.63–0.83], respectively, p < 0.0001), higher than NTproBNP and other echocardiographic parameters. Conclusions: Global PALS is associated with congestion and functional capacity in CA, suggesting its role as a more objective marker of disease severity in CA. Speckle tracking parameters may be used to enhance prognostic stratification in CA.