Background <p>Diabetes mellitus is a major public health challenge in South Africa, and poor glycaemic control contributes substantially to diabetes-related morbidity and mortality. National data describing temporal trends and geographic variation in HbA1c results within the public healthcare sector remain limited.</p> Methods <p>We conducted a retrospective cross-sectional analysis of routinely collected HbA1c test results from the National Health Laboratory Service between 1 January 2019 and 31 December 2024. Adults aged ≥ 18 years with valid sex and plausible HbA1c values (3.5–20.0%) were included. Very high HbA1c was defined as ≥9%. Temporal trends and geographic variation were examined descriptively, and multivariable binary logistic regression was used to identify independent predictors of HbA1c ≥ 9%.</p> Results <p>A total of 5,487,887 HbA1c test results were analysed. The mean age was 55.97 years (SD 14.73), and 64.5% of tests were from women. Overall, a substantial proportion of HbA1c results were in the very high range (≥9%), declining from 56.9% in 2019 to 36.3% in 2024 among adults undergoing testing in the public healthcare sector, indicating a shift in HbA1c distributions among individuals undergoing testing. Marked geographic variation was observed across provinces. In multivariable analysis, higher odds of HbA1c ≥ 9% were independently associated with male sex, older age (with peak risk in adults aged 45–59 years), province of testing, point-of-care HbA1c testing, and earlier calendar period.</p> Conclusions <p>Despite modest improvements over time, very high HbA1c levels remain common among adults undergoing testing in South Africa’s public healthcare sector. Persistent geographic and demographic disparities highlight the need for targeted, system-level interventions. Routine laboratory data provide a valuable platform for national surveillance and performance monitoring of glycaemic control.</p> Clinical Trial Number <p>Not applicable.</p>