Because of the increasing global attention being given to ecological security, the need to identify and quantify its underlying causes has sparked heated debate. However, there has not been any overall assessment of the ecological security in the G20 countries, which could seriously hinder sustainable development. Therefore, to investigate the ecological security situation in the G20 countries from 1999 to 2016 and facilitate the evaluation, three ecological indicators were introduced, namely ecological footprint (EF), ecological deficit or surplus, and eco-efficiency. To compare the driving factors of ecological security and provide a targeted reference for policymakers, the relationships between gross domestic product, population, energy consumption, technology, urbanization, and EF were then evaluated. To do so, the EF-Path-STIRPAT model was used, which is a combined path analysis and a STIRPAT model based on EF, and it was able to calculate the direct path elasticity coefficients between the variables to reveal the mechanisms and interactions. It was found that (a) the EF of developing countries is generally higher than that of developed countries in the G20, of which Russia has the highest EF, (b) from 1999 to 2016, the ecological status of the G20 gradually improved, and Argentina changed from an ecological deficit to an ecological surplus in 2005, and (c) the eco-efficiency of the G20 has been improved. China’s eco-efficiency increased significantly by 7.5% from 1999 to 2016, and the ecological environment improved significantly. According to the results of the EF-STIRPAT model, per capita, wealth should be increased steadily in developing countries, and developed countries should lie in the development of environmentally friendly high-tech industry and economic transformation to reduce energy intensity, improve their energy efficiency, and further optimize the energy structure.