Vascular stability and tone are maintained by contractile smooth muscle cells (VSMCs). However, injury-induced growth factors stimulate a contractile-synthetic phenotypic switch which promotes atherosclerosis and susceptibility to abdominal aortic aneurysm (AAA). As a regulator of embryonic VSMC differentiation, we hypothesised that Thymosin β4 may additionally function to maintain healthy vasculature and protect against disease throughout postnatal life. This was supported by identification of an interaction with Low density lipoprotein receptor related protein 1 (LRP1), an endocytic regulator of PDGF-BB signalling and VSMC proliferation. LRP1 variants have been identified by GWAS as major risk loci for AAA and coronary artery disease. Tβ4-null mice display aortic VSMC and elastin defects, phenocopying LRP1 mutants and suggesting compromised vascular integrity. We confirmed predisposition to disease in models of atherosclerosis and AAA. Diseased vessels and plaques were characterised by accelerated contractile-synthetic VSMC switching and augmented PDGFRβ signalling. In vitro, enhanced sensitivity to PDGF-BB, upon loss of Tβ4, coincided with dysregulated endocytosis, leading to increased recycling of LRP1-PDGFRβ and reduced lysosomal targeting. Our study identifies Tβ4 as a key regulator of LRP1 for maintaining vascular health, providing insight which may reveal useful therapeutic targets for modulation of VSMC phenotypic switching and disease progression.