Abstract Physical frailty is one of the fundamental hallmarks of aging. Older adults with mild cognitive impairment (MCI) are more likely to exhibit some characteristics of physical frailty such as worse gait and/or balance. Gait and/or balance disturbances may precede cognitive decline among cognitively healthy older adults. The purpose of this longitudinal observational study was to identify the temporal relationship between gait and/or balance disturbances and the onset of Alzheimer’s disease (AD) because it has not been well characterized among older adults with MCI. Data of 2,692 older adults with MCI (ages 74.5 ± 7.1 years, women making up 47% of the sample) were from the National Alzheimer’s Coordinating Center’s Uniform Data Set. Risk of incident AD by baseline gait and/or balance disturbances was examined by Cox regression models over 4.0 ± 3.0 years, adjusting for demographic characteristics, medical conditions, and study site where the data were collected. The presence of gait and/or balance disturbances predicted 38% to 55% (hazard ratios [HR] 1.38 to 1.55) increased risk of AD. Findings remained robust in sub-group analyses by sex and there were no noticeable changes compared with the original analyses. Thus, regardless of sex, gait and/or balance disturbances predicted the onset of AD among older adults with MCI. Health care providers may need to (1) catch a warning of possible cognitive decline by a frequent assessment of gait and/or balance disturbances and (2) provide effective strategies to improve gait and/or balance disturbances to potentially delay the onset of AD.