It has been observed that persons subsequently diagnosed with dementia often have difficulty completing commonly administered tests of executive function (EF). Interpretation of incompletion is problematic given the composite nature of EF tasks and the multiple impairments often demonstrated by persons with dementia. The goal of the present study was to determine the rate of failure to complete the Color-Word Stroop and Part B of the Trail-Making Test (TMT-B) in a clinical sample of persons with dementia and to explore neuropsychological predictors of incompletion. This study analyzed neuropsychological test data from 213 persons diagnosed with dementia at an interdisciplinary memory clinic. Index scores from the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) were examined as potential predictors of Color-Word Stroop and TMT-B completion in hierarchical logistic regression models. Rates of incompletion were high for both tests (60.6% for the Color-Word Stroop and 67.6% for the TMT-B). RBANS Language Index scores on the Color-Word Stroop predicted completion, while scores on the RBANS Visuospatial, Attention, and Immediate Memory indexes predicted TMT-B completion. The majority of the dementia sample was unable to complete the Color-Word Stroop and TMT-B executive tasks. Non-EF impairments may be implicated in completion of these tasks.