Loss to follow-up (LTFU) and death are unfavorable outcomes of human immunodeficiency virus (HIV) treatment. This study aimed to identify the predictors of LTFU and death among individuals with newly diagnosed HIV receiving dolutegravir (DTG)-based first-line antiretroviral treatment (ART) in eastern Ethiopia. A multisite prospective cohort study was carried out between October 2020 and July 2022. New case patients who started ART were enrolled consecutively and then followed up for the next 6 months. A structured questionnaire and checklists were used to collect data. HIV viral load was determined using the Abbott RealTime HIV-1 assay. Bivariable and multivariable logistic regression models were used to identify baseline factors associated with the outcomes. A total of 235 people with newly diagnosed HIV were enrolled; 16.6% (95% confidence interval, 12.3%-21.9%) were lost to follow-up, and 5.9% (3.5%-9.8%) died within 6 months of follow-up. Baseline World Health Organization clinical stage I (adjusted odds ratio, 3.93 [95% confidence interval, 1.34-11.57]), low viral load (3.67 [1.09-12.36]), and body weight (1.04 [1.01-1.07]) were predictors of LTFU, whereas nonfunctional status (10.02 [1.9-51.3]) was the only factor associated with death. LTFU and death rates among patients with DTG were relatively high, accounting for roughly a quarter of the attrition of people with newly diagnosed HIV from ART care and services. Thus, targeted interventions are required to reduce LTFU and death among individuals with HIV on ART. Further investigation is necessary to evaluate the long-term effects of DTG-based regimens on LTFU and its impact on HIV mortality rates, and qualitative research, specifically tracing LTFU, is recommended.