Cognitive impairment poses a major public health challenge in aging societies, and dietary nutrition is considered a key modifiable target. The China-adapted MIND (cMIND) diet has been preliminarily established, but longitudinal evidence supporting its association with cognitive impairment among older adults remains limited. This study focused on Chinese older adults aged ≥65 years and used four waves of data (2008–2018) from the Chinese Longitudinal Healthy Longevity Survey (CLHLS) to identify long-term trajectories of cMIND diet adherence through latent class growth analysis and examine their relationship with cognitive impairment. Results showed that cMIND diet adherence among older adults followed four distinct trajectories: persistently high, stable moderate, moderate-declining, and persistently low. Compared with the persistently high adherence group, the risk of cognitive impairment was 35% higher in the moderate-declining group, and 48% higher in the persistently low group (both P<0.05). Advanced age, educational level below elementary school, rural residence, lack of regular physical exercise, and depression were independent risk factors for low-adherence trajectories. Maintaining long-term high adherence to the cMIND diet, rather than focusing solely on one-time intake, is an important modifiable target for preventing cognitive impairment among older adults. Community-based nutritional interventions should be initiated early and sustained over the long-term, with particular attention to vulnerable populations, including rural residents and those with low educational levels.