A floristic study of Eastern Mongolian Steppes has been conducted during the growing season of 2014 and continued in 2017, when we focused more on the Numrug Strictly Protected Area. Major objectives of this study were to assess floristic diversity of Eastern Mongolian Steppes and to review the conservation status of some endangered, rare, and very rare species. A total of 488 taxa of vascular plants, which belong to 254 genera of 75 families, including three endemic and 16 subendemic species were sampled. Among these were three endangered, three vulnerable, five near threatened, 16 very rare, and 39 rare species. Remarkably, Myriophyllum sibiricum and Typha latifolia are newly recorded for the flora of Mongolia. Moreover, new distribution records of some species were noted in four phytogeographical regions of Eastern Mongolia, such as Foothills of Great Khingan (30), East Mongolia (12), Mongolian Dauria (11), and Middle Khalkha (3). Finally, we assessed regional conservation status of the five species (two endangered and three vulnerable) from the East Mongolia using the International Union for Conservation of Nature Red List criteria and categories.