Background: A source of bias in surrogate decision-making has been articulated alternatively as the false consensus effect, assumed similarity, and attributive/social projection by which proxies overestimate the degree that their values and preferences are shared by others, resulting in a projection of their own beliefs onto others in the process of surrogate decision-making. The aims of this study are to assess for projection of proxies’ preferences onto surrogate assessments of everyday preferences for persons with cognitive impairment (CI) and to address cognitive and neuropsychiatric symptom burden as predictors of projection. Method: The sample included 116 dyads of persons with CI (Clinical Dementia Rating Scale score ≥ 0.5) and their care partners. The Preferences for Everyday Living Inventory (PELI) was used to assess importance of preferences among persons with CI. Care partners completed two separate PELI assessments: one from the perspective of the persons with CI (i.e., acting as a surrogate decision-maker) and one from their own perspective. To assess for projection of care partners’ preferences onto surrogate assessments of preferences for persons with CI, two-step regression with multivariable-adjusted general linear models was used. Result: Significant projection effects were noted within the PELI domains of autonomous choice, personal growth, and keeping a routine (p < 0.0005). More significant cognitive impairment was associated with increased projection effects within the PELI domains of autonomous choice and personal growth (p < 0.01). Conclusion: The results of this study suggest that projection effects of care partners own preferences may be a significant source of bias in everyday proxy decision-making for persons with CI, particularly for those with more significant CI.