Measuring physical activity (PA) in people with mild cognitive impairment or dementia can be difficult. The aim was to investigate the validity and acceptability of three different PA measurement methods. The mixed-method analysis included 49 participants with mild cognitive impairment or dementia, who completed a daily calendar recording PA, the International Physical Activity Questionnaire, and the Longitudinal Aging Study Amsterdam PA Questionnaire, and those who wore a Misfit Shine accelerometer. The quantitative analysis showed equal completion rates for the International Physical Activity Questionnaire and the accelerometer but a lower completion rate for the calendar. Correlations between outcome measures were moderate or strong. The qualitative analysis indicated that all measures were acceptable, though some participants required help to complete the calendars or fasten the accelerometers. The study supported the validity of these methods for people with mild cognitive impairment and mild dementia. Using accelerometers and completing calendars might increase the motivation to be active for some people.