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BACKGROUND Motor and gait disturbances are evident in early Alzheimer and non-Alzheimer dementias and may predict the likelihood of mild cognitive impairment (MCI) or progression to dementia. OBJECTIVE We investigated the Timed-Up-and-Go (TUG) measure of functional mobility in predicting cognitive decline and incident MCI or early dementia (MCI-dementia). DESIGN Prospective cohort study with 4.5 years follow-up. SETTING Population based. PARTICIPANTS 2,544 community-dwelling older adults aged 55+ years. METHODS Participants with baseline data on TUG, fast gait speed (GS), knee extension strength (KES) and performance-oriented mobility assessment (POMA) gait and balance were followed up for cognitive decline (Mini-Mental State Exam; MMSE drop of ≥2, among 1,336 dementia-free participants) and incident MCI-dementia (among 1,208 cognitively normal participants). Odds ratio (OR) and 95% confidence intervals (95% CI) were adjusted for age, sex, education, smoking, physical, social and productive activity, multi-morbidity, metabolic syndrome and MMSE. RESULTS Per standard deviation increase in TUG, POMA, GS and KES were significantly associated with incident MCI-dementia: TUG (OR = 2.84, 95% CI = 2.02-3.99), GS (OR = 2.17, 95% CI = 1.62-2.91), POMA (OR = 1.88, 95% CI = 1.22-2.92) and KES (OR = 1.52, 95% CI = 1.15-2.02). Adjusted OR remained significant only for TUG (OR = 1.52, 95% CI = 1.01-2.31) and GS (OR = 1.53, 95% CI = 1.08-2.16). Areas under the curve (AUC) for TUG (AUC = 0.729, 95% CI = 0.671-0.787) were significantly greater than GS (AUC = 0.683, 95% CI = 0.619-0.746), KES (AUC = 0.624, 95% CI = 0.558-0.689) and POMA (AUC = 0.561, 95% CI = 0.485-0.637). Similar associations with cognitive decline were significant though less pronounced, and adjusted ORs remained significant for TUG, GS and POMA. CONCLUSION Functional mobility decline precedes incident MCI and early dementia. The TUG appears to be especially accurate in predicting the future risks of adverse cognitive outcomes. TRIAL REGISTRATION ClinicalTrials.gov NCT03405675. Registered 23 January 2018 (retrospectively registered).