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 in older adults is lacking. 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 via latent class growth analysis and to examine their relationship with cognitive impairment. Results showed that cMIND diet adherence in older adults followed four distinct trajectories: persistently high, stable moderate, moderate-declining, and persistently low. Compared with the persistently high adherence group, the moderate-declining group had a 35% higher risk of cognitive impairment, and the persistently low group had a 48% higher risk (both P < 0.05). Advanced age, illiteracy, 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 single?time?point intake, is an important modifiable target for preventing cognitive impairment in older adults. Community-based nutritional interventions should be initiated early and sustained, with particular attention to vulnerable populations such as those in rural areas and with low educational levels.