Introduction: Loneliness in older persons with cognitive impairment (PCI) may beget loneliness in their family caregivers, depending on buffering resources caregivers possess. This study examined the association between loneliness in older PCI and loneliness experienced by their family caregivers, and the moderating role of caregiver mastery in this association. Methods: Dyadic data from 135 PCI and their family caregivers in Singapore were analyzed using multivariable regression. Loneliness was measured using a three-item UCLA loneliness scale. Mastery was assessed using a seven-item Pearlin instrument. Results: Multivariable regression showed that PCI loneliness and caregiver loneliness were weakly associated, taking other covariates into account. Notably, a significant interaction between PCI loneliness and caregiver mastery was observed, indicating that PCI loneliness was associated with caregiver loneliness only when caregivers had low mastery. Conclusion: Lonely PCI may share their feelings of loneliness with their caregivers, and this can lead to loneliness among caregivers if they have low mastery. Promoting caregiver mastery may help reduce caregiver loneliness, directly and indirectly as a buffer against PCI loneliness.