Abstract Background Despite the well‐established link between type 2 diabetes and dementia, its impact on the prodromal dementia phase remains controversial, as does the impact of comorbid cardiovascular disease (CVD). In this study, we assessed the impact of diabetes and CVD on the development of cognitive impairment no dementia (CIND) and its progression to dementia. Methods In the Swedish National Study on Aging and Care‐Kungsholmen (SNAC‐K), a cohort of cognitively‐intact individuals (n=1840) and a cohort of individuals with CIND (n=682) aged ≥60 years were followed over 15 years. At baseline and each follow‐up (every 3 or 6 years), a neuropsychological test battery was administered to assess five cognitive domains (episodic memory, processing speed, executive function, verbal fluency, visuospatial abilities). CIND was defined as having no dementia and cognitive performance ≥1.5 SDs below age group‐specific means in at least one domain. Dementia was diagnosed according to international criteria. Diabetes (controlled and uncontrolled: HbA1c <7.5% vs. ≥7.5%) was assessed based on medical history, clinical records, and glycated hemoglobin. CVD (atrial fibrillation, heart failure, ischemic heart disease, cardiac valve diseases, and bradycardias) was ascertained through medical examinations and medical records. Data were analyzed with multivariable Cox regression models. Results At baseline, 135 (7%) participants in the cognitively‐intact cohort and 85 (12%) in the CIND cohort had diabetes. During follow‐up (mean 9.2 ± 3.1 years 2.2–15.6 years), 544 (30%) participants in the cognitively‐intact cohort developed CIND. Diabetes was associated with a 35% higher risk of CIND (HR 1.35, 95% CI: 0.98‐1.88) compared to the diabetes‐free group. This risk rose to 75% in people with comorbid diabetes and CVD (HR 1.75, 95% CI: 1.01‐3.04) and was doubled in people with uncontrolled diabetes (HR 1.97, 95% CI: 1.12‐3.49). In the CIND cohort, 151 (22%) individuals progressed to dementia during follow‐up. Participants with uncontrolled diabetes had triple the risk of progressing to dementia (HR 3.00, 95% CI: 1.26‐7.13) vs. the diabetes‐free group, and the HR of dementia was 4.36 (95% CI: 1.51‐12.59) in individuals with uncontrolled diabetes and comorbid CVD. Conclusions Uncontrolled diabetes increases the risk of cognitive impairment and accelerates its progression to dementia, particularly in older adults with comorbid CVD.