Mongolia has a low incidence of human immunodeficiency virus (HIV) infection, with 281 cases reported at the end of 2019 and an estimated incidence rate of <0.01 cases per 1000 population. However, no study has analyzed the association between antiretroviral therapy (ART) outcomes and pretreatment characteristics of patients with HIV/acquired immunodeficiency syndrome (AIDS) in Mongolia. This retrospective study aimed to determine the survival, CD4 T cell recovery, and immune reconstitution pattern during ART in HIV patients and to determine baseline patient characteristics associated with ART outcomes. Based on three different World Health Organization (WHO) guidelines, we analyzed the 3-year observation data of 166 patients with HIV/AIDS who received treatment between 2010 and 2017. An increase of >50 cells/μL indicated CD4 T cell count recovery, and a cell count of ≥500 cells/μL in patients with a baseline cell count of <500 cells/μL indicated immune reconstitution. In this study, the 3- and 1-year mortality rates were 5.4% (survival rate: 94.6%) and 3.6%, respectively. A total of 83% of deaths that occurred in the observation time occurred within the first 3 months. The CD4 T cell count recovery rates at 3, 12, and 36 months were 62.7%, 80.7%, and 89.2%, respectively. The CD4 T cell count increased to >500 cells/μL in 95 of 145 (65.5%) patients with a baseline cell count of <500 cells/μL after 36 months of ART. The baseline CD4 T cell count was found to be a sensitive indicator for immune reconstitution. An advanced pretreatment clinical stage of HIV infection (as classified by the WHO classification), a low CD4 T cell count in the peripheral blood, and a high viral load before the initiation of the first-line ART accurately predicted survival, CD4 T cell count recovery, and immune reconstitution in Mongolian patients with HIV/AIDS.