WMH shape is associated with long-term risk for dementia after 10 years in community-dwelling older adults. The current study aimed to investigate the association of WMH shape and decline in three cognitive domains over five years' time in community-dwelling older adults. The association of baseline WMH shape (solidity, convexity, concavity index, fractal dimension, and eccentricity) and cognitive decline over 5.2 ± 0.3 years (domains: memory, executive function, and processing speed) was investigated using linear regression models in the Age, Gene/Environment Susceptibility-Reykjavik (AGES) study (n = 2493). A more irregular shape of periventricular/confluent WMH was related to cognitive decline in the memory domain, the executive function domain, and the processing speed domain over five years (p < 0.05). No associations were found between deep WMH shape and decline in the cognitive domains. These findings show that WMH shape patterns may be indicative of relatively short-term cognitive decline in community-dwelling older adults. This supports the evidence of WMH shape being a valuable marker that may be used to assess and predict cognitive outcome related to cerebrovascular disease progression.