Background: It is unclear whether the limitations of young persons with a mild intellectual disability or borderline intellectual functioning preclude feasibility of the daily diary method. Method: For 60 consecutive days, 50 participants (M[subscript age] = 21.4, 56% male) who receive care in an ambulatory, residential, or juvenile detention setting, self-rated both standardised and personalised diary questions through an app. Diary entries were used for feedback in treatment. Interviews were used to explore acceptability. Results: Average compliance was 70.4%, while 26% of participants dropped out. Compliance was good in ambulatory (88.9%) and residential care (75.6%), but not in the juvenile detention setting (19.4%). The content of self-selected diary items varied widely. Participants deemed the method acceptable. Conclusions: Daily monitoring is feasible for individuals with a mild intellectual disability or borderline intellectual functioning receiving ambulatory or residential care, and can provide scientists and practitioners with important insights into day-to-day behavioural patterns.