Insulin resistance (IR) is a prevalent metabolic condition characterized by reduced cellular responsiveness to insulin and consequent hyperinsulinemia, and it is a key component of type 2 diabetes and metabolic syndrome. A growing body of evidence suggests IR is a critical accomplice in the pathogenesis of various cancers. This review synthesizes evidence on underlying molecular mechanisms, including the integrated roles of the insulin/IGF system, chronic inflammation, metabolic reprogramming, and mitochondrial dysfunction. This review proposes that metabolic dysregulation should be viewed as a modifiable oncogenic force. This perspective illuminates new pathways for understanding cancer development and offers promising avenues for prevention and therapeutic intervention.