Energy sustainability is the main motive behind the evolution of the concept of well integrity in the oil and gas industry. The concept of well integrity adopts technical, operational, environmental, organizational, and safety measurements to secure the energy supply throughout the life of the well. Technically, a high quality well performance can be maintained by establishing robust barrier systems that are responsible for preventing, controlling, and mitigating potential risks that could arise during the well life cycle. A barrier system is conventionally nested from one or multiple elements that act individually or collectively to scaffold the well integrity. The protection layers in a wellbore can be lost if the integrity of the barrier system is compromised according to the failure of one or all of its elements. Failure can be triggered by technical or non-technical factors. In this study, technical aspects that drive barrier failure mechanisms have given more emphasis. The failure mechanisms of the key mechanical barrier systems, such as casing strings, cement, diverters, blowout preventers (BOPs), production stream valves, and seal assemblies, have been thoroughly investigated. In this study, a comprehensive review of barriers failure mechanisms has been conducted to identify the roots of failures and to outline some of the essential safety measures adopted to avoid the loss of well control. The major findings of this paper revealed that well barrier systems are highly susceptible to failure in unconventional reservoirs, deep and ultra-deep offshore wells, and geothermal wells. The predominant failures identified are casing collapse resulting from cyclic loads, cement percolation by gas migration, cement carking by hoop stress, BOPs wear and tear promoted by frequent tests, and elastomeric materials disintegration caused by acidic gases. Considering these failure mechanisms while designing a wellbore can help the engineers improve the construction quality. In addition, it can assist the operation and maintenance crews in optimizing safe operation boundaries.