Background: Prosthetic graft replacement of thoracic aortic aneurysms (TAA) yields benefits but risks persist in the native aorta, especially in Marfan syndrome (MFS). Differential biomechanics between chronically grafted and native aortic regions are unknown. Methods: Functional cardiac magnetic resonance (cine-CMR) imaging was performed in non-surgical MFS patients (root or ascending diameter≤4.5cm) and patients after (>1 year) proximal grafting. Analyses included mid-ascending and -descending aortic size (diameter, area) and compliance indices, including global circumferential strain (GCS), fractional area change (FAC), stiffness index, and distensibility. Results: 46 MFS patients underwent cine-CMR, including 21 with chronic proximal grafts (10.5 +/- 7.3 years post-operatively). Patients with and without grafts had similar clinical and hemodynamic characteristics. Grafted and non-grafted ascending aortic size was similar between groups (p=NS), but functional parameters differed as evidenced by decrements in GCS, FAC, stiffness index, and distensibility (all p<0.001), consistent with prosthesis-associated reductions in compliance: Proximal grafts associated with decreased ascending aortic strain (B= -7.09, p<0.001) independent of age and aortic root size. Notably, native descending aortic size was larger in post-operative patients (p<0.01), paralleled by increased GCS (p<0.001) and decreased stiffness (p=0.04). In multivariate analysis, proximal grafts associated with increased descending aortic strain when controlling for ascending aortic area (B=4.19, p<0.001) or root size (B=3.14, p=0.045). Conclusions: Marfan syndrome patients with chronic proximal aortic grafts manifest distinct vessel wall biomechanics in grafted and native regions that differ from non-surgical comparators, including decreased strain (a marker of reduced compliance) within grafted territories and increased strain in native aortic regions distal to grafts.