Activities of daily living can be affected by cognitive decline. Self-report measurement of functioning is attractive due to ease of data collection, low cost, and accessibility via technology-assisted means, and for understanding patient perspective. A concern is with reliability of such measurement as cognitive decline occurs. We compared a widely used, self-report “legacy” measure of functioning, Lawton and Brody’s Instrumental Activities of Daily Living Scale (IADLS), with a subset of physical functioning items from the Patient-Reported Outcomes Measurement Information System (PROMIS). The study sample consisted of 304 individuals of varying cognitive status: normal, mild cognitive impairment (MCI), or early dementia. An expert consensus method was used to select PROMIS functional items most relevant to neurocognitive disorder and to identify major functional sub-domains. Selected PROMIS functional subscales and the IADLS were then evaluated with respect to cognitive status. Few PROMIS functional items were useful in identifying MCI, while we reaffirmed the utility of the IADLS. Also, even mild depression levels were found to have negative effects on functioning according to both PROMIS and IADLS.