Abstract Malnutrition is highly prevalent in older persons with dementia. Therefore, strong predictors of malnutrition in this population are crucial to initiating early interventions. This study evaluates the association between the probability of having malnutrition with the muscle volume and intramuscular fat (iMAT) of the masseter and the tongue in magnetic resonance imaging (MRI) of community-dwelling older persons diagnosed with mild dementia followed up for 5 years. This is a longitudinal study conducted in the western part of Norway. Muscle volume and iMAT of the tongue and masseter were computed from structural head MRI obtained from 65 participants of the Dementia Study of Western Norway using Slice-O-Matic software for segmentation. Malnutrition was assessed using the Global Leadership Initiative on Malnutrition Index. Linear mixed models were conducted. Having malnutrition at baseline was associated with lower muscle volume (odds ratio [OR] 0.60, standard error [SE] 0.20; p = .010) and higher iMAT (OR 3.31, SE 0.46; p = .010) in the tongue. At 5 years follow-up, those with lower muscle volume (OR 0.55, SE 0.20; p = .002) and higher iMAT (OR 2.52, SE 0.40; p = .022) in the tongue had a higher probability of presenting malnutrition. The masseter iMAT and volume were not associated with malnutrition in any of the adjusted models. In people diagnosed with mild dementia, tongue muscle volume and iMAT were associated with baseline malnutrition and the probability of developing malnutrition in a 5-year trajectory. In the masseter, there were no significant associations after adjustments.