The rapid development of modern electronic technology has provided high efficiency for social production, but it also brings serious electromagnetic interference (EMI). Although traditional metals and their alloys can serve as good electromagnetic shielding materials, their heavy weight, high cost as well as poor corrosion resistance limit their application in EMI shielding. Therefore, electromagnetic shielding materials with lightweight are gradually attracting more and more attention to meet the trend of lightweight and highly integrated electronic equipment. Polymers are widely used in EMI shielding materials because of their unique characteristics, such as lightweight, high flexibility and excellent corrosion resistance. This review is focused on the well-studied polymer-based lightweight composites, with the aim of providing a comprehensive review of the existing EMI shielding mechanism, preparation and application of polymer-based lightweight composites, along with retrieving new general insights into the advantages and limitations associated with the different measurement and prediction techniques, as well as the EMI shielding effectiveness of polymer-based lightweight composites including metal-containing composites, intrinsic conductive polymers, filler-containing polymers and newly discovered materials. These lightweight materials with fine EMI shielding performance will find more and more potential applications in communications, electronics, aerospace, military, environmental protection, etc.