Functional disability in older persons with cognitive impairment is associated with reduced quality of life and greater mortality, health care utilization, and caregiver burden. Episodic memory, executive function, apathy, depressive symptoms, and medical burden have been identified as cross-sectional predictors of functional disability but have received little longitudinal investigation in a way that explicates how changes in these variables relates to functional disability. Functional disability also drives the distinction between the diagnoses of Mild Cognitive Impairment (MCI) and dementia; however, little is known about the rates of functional decline in these groups over time. This study utilized multi-level modeling to determine the longitudinal associations between episodic memory, executive function, apathy, depressive symptoms, and medical burden and functional decline in older persons with MCI and two of the most prevalent types of dementia, Alzheimer’s disease and vascular dementia. Results provide support for the longitudinal associations between memory, executive function, and apathy symptoms and instrumental activities of daily living (IADL) performance. Alzheimer’s disease was associated with a faster rate of function decline than normal aging and vascular dementia, but a rate not significantly different than seen in MCI. Longitudinal decline in IADLs was non-significant in both normal aging and vascular dementia.