When compared with controls, both mild cognitive impairment (MCI) and dementia are each associated with impaired memory for future intentions, or prospective memory (PM). However, prior studies have failed to agree on whether there are group differences in PM function between those with MCI and dementia. Furthermore, the degree and nature of the impairment remains to be clarified, as does the degree to which this impairment is secondary to deficits in other aspects of cognition. In the present study, MCI (n = 48), dementia (n = 39), and control participants (n = 53) were compared on Virtual Week, a measure that closely represents the types of PM tasks that occur in everyday life. Both clinical groups exhibited impairment irrespective of the specific task demands, but the magnitude of this deficit was greater for those with dementia. After covarying for other key cognitive parameters, although the absolute magnitude of the deficit was reduced, significant impairment remained. These results indicate that individuals with MCI, and to a greater extent dementia, experience generalized difficulties with PM. It is suggested that, while other cognitive deficits contribute to these difficulties, there is something unique to prospective remembering that may be additionally disrupted in these groups.