Ecological security is the comprehensive characterization of the noosystem’s overall collaborative capacity in relation to human welfare within a whole society. Recent years, however, have witnessed the excessive exploitation of natural resources via anthropogenic activities that have consistently triggered or accelerated ecological deterioration. In addition, an adequate ecological security assessment has yet to be conducted in a steppe ecoregion, especially when considering that the steppe is the major ecoregion category in China. In this study, we selected a typical steppe ecoregion in Inner Mongolia, northern China, as a representative region to conduct a spatiotemporal assessment of ecological security from both global and local perspectives. Along with an evaluation indicator system constituting 25 separate indicators covering three different systems (society, economy, and nature), we applied an improved grey target decision-making method in the initial indicator conversion process. As it pertains to weight determination, we employed a weight calculation method that weighed all indicators synthetically. Research findings indicated that a progressive deterioration of ecological security has been observed in the typical steppe ecoregion of China from 1990 to 2014, with a decline in the security situation and even more severe deterioration observed from a local perspective than a global perspective. Social and economic systems, specifically an excessive rural population and unreasonable development (mainly grazing and mining), were the major factors that promoted ecological security deterioration, while the natural system has contributed only slightly to this deterioration.