In the past few decades, waste generation has increased significantly in India, as well as across the globe. Therefore, to avoid the acquisition of new landfill sites, vertical expansion of existing municipal solid waste (MSW) landfills is a requirement of the present scenario. Owing to the heterogeneous nature of waste and varying degrees of decomposition in MSW, the conventional practices to calculate safety margins may not be suitable to handle the variability of the design parameters. Therefore, a system reliability framework is proposed for the optimum design of mechanically stabilized earth (MSE) retaining walls against external failure modes. The properties associated with MSW landfills, their foundation soil, and MSE walls are considered as random variables. The overturning, sliding, and bearing capacity failure modes are considered using a three-part wedge mechanism. The design charts are provided to obtain the adequate length of the reinforcement and the dimensions of the MSE wall. For satisfactory performance of the MSE wall, the minimum ratio of the length of the reinforcement to the height of the MSE wall should be 0.55 to 0.6 when an additional waste mass is placed on the existing height of the landfill. © 2020 American Society of Civil Engineers.