Pro-resolving and anti-inflammatory mediator products of murine 12/15-lipoxygenase (LOX) exhibit potent actions on vascular inflammation and protect against the progression of atherosclerosis. The present study was designed to determine whether augmenting dietary lipids modulates the body’s endogenous anti-inflammatory pro-resolving mechanisms and promotes atherosclerosis. We investigated the biometabolic consequences of variations in lipid mediator biosynthesis using genetic knockout and overexpression models of 12/15-LOX mice fed the commonly used ‘Western diet’. Unexpectedly, this high-fat diet annulled the protective actions of 12/15-LOX, and the combination of a Western diet and 12/15-LOX overexpression paradoxically promoted inflammation leading to production of diet-related and 12/15-LOX-dependent blood mediators that differentially activated endothelial cells via expression of ICAM-1. Hyperlipidemia not only affected the biosynthesis of lipoxin A 4, a key pro-resolving mediator, but also disrupted the protective pro-resolving function of 12/15-LOX products, and the enzyme pathway no longer protected against atherosclerosis in vivo. We uncovered a novel mechanism whereby a high-fat diet as well as hyperlipidemia disrupt the homeostasis of inflammation resolution. These findings underscore the importance of dietary essential PUFAs and LOX-derived lipid mediators in combination with lipid-lowering agents in the prevention and treatment of atherosclerotic cardiovascular diseases.