Namibia has a widespread human immunodeficiency virus (HIV) epidemic, with HIV spread mostly through heterosexual transmission. Although the number of persons receiving antiretroviral therapy (ART) has increased, the effectiveness of the 90-90-90 methods for testing, treatment, and suppression has not been assessed. Furthermore, identifying parameters linked with treatment adherence and viral suppression will assist in creating appropriate expedited interventions. However, ignoring sample selection when estimating treatment adherence and viral load may result in misleading estimates. We fit a joint distributional model of ART treatment adherence and viral load, to adjust for sample selection bias among HIV patients on ART in Namibia, examine the presence of tail dependence in sample selection bias, and investigate the factors associated with viral load and ART adherence. The study used data from HIV patients, from January 1, 2010, to December 31, 2015, aged above 16 years, on ART in four health facilities in the Erongo region, Namibia. A Heckman-type selection analysis was carried out using families of copulas, i.e., Normal, Frank, FGM, AMH, Student-t, and the 0-, 90-, 180-, and 270-degree rotated versions of Joe, Gumbel, and Clayton, to capture dependence in the outcomes. Models were selected based on the lowest AIC and BIC. The results showed a strong negative correlation between adherence to ART treatment and viral load suppression, with adherence of \(>\) 95% associated with viral suppression at 6 and 12 months. Findings further showed that current body weight was statistically significant with viral suppression. However, demographic and clinical characteristics were not associated with ART adherence or viral suppression. Similar studies in the future need to consider socio-economic factors in addition to those considered in this study.