Hypertension is a poorly controlled risk factor for cardiovascular disease in South Africa, particularly among patients receiving care in the public sector who are mostly from low socioeconomic backgrounds. This cross-sectional study investigated uncontrolled hypertension, treatment, and predictors among hypertensive out-patients attending primary health care facilities in Johannesburg, South Africa. The WHO STEPwise approach to the surveillance of non-communicable diseases was used to collect data, including sociodemographic and lifestyle factors, health status, and measurements for anthropometry and blood pressure along with self-reported adherence to treatment, estimated through the general medication adherence scale. Uncontrolled hypertension was defined as systolic blood pressure ≥140 mmHg and/or diastolic blood pressure ≥90 mmHg in diagnosed patients receiving anti-hypertensive treatment. Overweight and obesity were defined as a body mass index ≥25 and ≥30 kg/m², respectively. Logistic regression models were used to assess the predictors of uncontrolled hypertension. Four hundred (n = 400) hypertensive out-patients (mean age: 50 ± 8 years) participated in this study, with most living in poor sociodemographic environments. The prevalence rate of uncontrolled hypertension was 57%. Obesity (62% vs. 42%, p ≤ 0.0001), salt consumption (90% vs. 55%, p ≤ 0.0001), alcohol intake (42% vs. 19%, p ≤ 0.0001), a smoking habit (23% vs. 4%, p ≤ 0.0001), alternative medicine use (51% vs. 40%, p = 0.043), and comorbidities (64% vs. 36%, p ≤ 0.0001) were higher in the uncontrolled group than the controlled group, whereas the prevalence of physical activity (38% vs. 15%, p ≤ 0.0001) was high in the controlled group vs. the uncontrolled. Overall, 85% of the patients moderately adhered to treatment, only 2% exhibited high adherence, and 13% demonstrated low adherence; over half of the patients received tri-therapy treatment. The predictors of uncontrolled hypertension are a number of prescribed antihypertensive therapies [adjusted odds ration = 2.39; 95% confidence interval: 1.48-3.87], treatment adherence [0.46; 0.21-0.97], salt consumption [28.35; 7.87-102.04], physical activity [0.22; 0.13-0.37], current alcohol use [2.10; 1.22-3.61], and current cigarette smoking [4.79; 1.88-12.18]. The high prevalence of uncontrolled hypertension in this study suggests a need to optimize prescriptions, adherence to BP-lowering medications, and lifestyle modifications. The management of comorbidities such as diabetes could offer considerable benefits in controlling blood pressure.