Introduction: Primary health care services are focused on promotion, prevention, and rehabilitation, in addition to implementing various health programs. Mental workload is defined as mental, cognitive, or intellectual demands experienced by workers during their workday. Objectives: To describe mental workload according to sociodemographic and occupational characteristics of professionals responsible for health programs in primary care facilities in the Ñuble region, Chile. Methods: A descriptive, cross-sectional quantitative study was conducted with a sample of 69 professionals. Data were collected through surveys assessing sociodemographic and occupational characteristics, workplace environmental conditions, and the Escala Subjetiva de Carga Mental de Trabajo (Subjective Mental Workload Scale), validated for use in Chile. Results: The mental workload was moderate-to-high, with a mean score of 3.68. The dimensions “cognitive demands and task complexity” and “task characteristics” presented the highest mean scores, while “time organization” and “work pace” showed the lowest mean scores among the variables studied. Conclusions: Younger workers with fixed-term contracts, who perceived their work environment conditions as highly inadequate and were in charge of more than 16 people presented the highest levels of mental workload.