Abstract Aims To examine associations of diastolic and systolic blood pressure (SBP) at age 50, 60, and 70 years with incidence of dementia, and whether cardiovascular disease (CVD) over the follow-up mediates this association. Methods and results Systolic and diastolic blood pressure were measured on 8639 persons (32.5% women) from the Whitehall II cohort study in 1985, 1991, 1997, and 2003. Incidence of dementia (n dementia/n total = 385/8639) was ascertained from electronic health records followed-up until 2017. Cubic splines using continuous blood pressure measures suggested SBP ≥130 mmHg at age 50 but not at age 60 or 70 was associated with increased risk of dementia, confirmed in Cox regression analyses adjusted for sociodemographic factors, health behaviours, and time varying chronic conditions hazard ratio (HR) 1.38; 95% confidence interval (95% CI) 1.11, 1.70. Diastolic blood pressure was not associated with dementia. Participants with longer exposure to hypertension (SBP ≥ 130 mmHg) between mean ages of 45 and 61 years had an increased risk of dementia compared to those with no or low exposure to hypertension (HR 1.29, 95% CI 1.00, 1.66). In multi-state models, SBP ≥ 130 mmHg at 50 years of age was associated with greater risk of dementia in those free of CVD over the follow-up (HR 1.47, 95% CI 1.15, 1.87). Conclusion Systolic blood pressure ≥130 mmHg at age 50, below the conventional ≥140 mmHg threshold used to define hypertension, is associated with increased risk of dementia; in these persons this excess risk is independent of CVD.