Person-centred care requires improved documentation of nursing home resident wellbeing, e.g. by nursing staff proxy assessments. Previous studies mainly focused on proxy self-report agreement of quality of life of people with dementia, using lengthy questionnaires. This is the first study to investigate how well nursing staff assess residents’ wellbeing after training, using a single-question assessment method of happiness and engagement. We conducted a cross-sectional mixed-method study, including proxy assessments from 49 nursing staff, and self-reports from 49 nursing home residents without dementia (mean age 85). We explored agreement between colleagues, and between proxy assessments and self-reports, and potential nursing staff characteristics associated to this (age, experience, hours worked per week). Brief written motivations were evaluated on nursing staffs’ understanding of the happiness and engagement concepts. The results showed low agreement between colleagues, and low agreement between proxy assessments and self-reports. Nursing staff assessed happiness and engagement substantially higher than residents’ self-reports. Hours worked per week was related to happiness proxy assessments, but none of the included nursing staff characteristics were related to proxy self-report agreement. Nursing staff interpreted the concepts in diverse ways. Overestimating resident wellbeing when using this single-assessment method may undermine subsequent efforts to improve wellbeing. We could not identify which nursing staff could best provide wellbeing assessments. For now, proxy wellbeing assessments should always be combined with regular self-reports whenever possible.