Cardiovascular diseases represent the leading cause of death, and the underlying cause of most cardiovascular deaths is atherosclerosis. Atherosclerosis is a progressive disease in which arteries accumulate lipid-laden plaques while progression of atherosclerosis is influenced by a variety of factors such as age, gender, and life-style choices. Notably, one unifying attribute of atherosclerotic risk factors is their connection to vascular stiffening which is a clinical predictor of cardiovascular risk. Intimal stiffening is now understood to contribute the atherosclerosis progression by disrupting endothelial barrier integrity. Endothelial cells respond to intimal stiffening by increasing their actomyosin contractility which disrupts cell-cell junctions and leads to enhanced endothelial permeability and leukocyte transmigration, hallmarks of atherosclerosis. However, intimal stiffening has recently been shown to be spatially heterogeneous and characterized by greater point-to-point heterogeneity. The impact of intimal stiffness heterogeneities on proximal and distal barrier integrity or atherogenesis is not well studied. Identifying how endothelial cells respond to heterogenous matrix rigidity will provide insight into how endothelial cells interpret complex mechanical cues and how complex mechanical heterogeneities will contribute to atherogenesis. Understanding these biological questions may help guide the development of therapies to treat the cellular response to intimal stiffness cues to slow the progression of atherosclerosis. To investigate the response of endothelial monolayers to heterogeneous matrix rigidity, I designed and fabricated models that incorporate flexible micropillars to geometrically control matrix stiffness and present complex mechanical cues. I identified that endothelial cells simultaneously contacting both stiff and compliant matrix adopt a pro-atherogenic phenotype identical to uniformly stiff substrates characterized by disrupted cell-cell junctions and enhanced leukocyte transmigration. Importantly, subcellular metrics of actomyosin contractility indicated that cells within the monolayer did not retain the ability to respond to local matrix cues. Motivated by these results, I developed micropillar models that could be used to measure the range at which matrix stiffness heterogeneities could impact endothelial phenotype. Micropillar models that presented discrete transitions in matrix stiffness created long-ranged oscillatory patterns of endothelial barrier integrity and cell contractility, illuminating a novel biophysical phenomenon in which stiffness-mediated cell-matrix interactions regulate cell-cell interactions at a considerable distance. Collectively, these two studies identified how endothelial monolayers may respond to age-related heterogeneous intimal stiffening during atherogenesis including the impact on both proximal and distal cells within the endothelium. I propose that future studies seeking to mitigate the progression of atherosclerosis may benefit from targeting the long-ranged impacts of age-related heterogeneous intimal stiffening.