Background This study of age and gender profiles of health care populations fills a gap in the research literature by providing a population study of both single health agency and inter-agency ‘Shared Care’ populations. Methods It combines anonymized data to link individual cases across Community Health (N = 82 751), Mental Health (N = 19 029) and Social Services (N = 19 461) populations in one county Health Authority (N = 646 239) over 3 years. It compares age and gender characteristics of single care populations and overlapping inter-agency ‘Shared Care’populations. Results Approximately two-thirds of all care populations were female compared to half (513.1) in the general Health Authority population. These differences were accentuated for almost all inter-agency ‘Shared Care’ populations, where, whilst a younger care profile emerged for mental health and social services dual agency clients, for other shared populations a distinct care profile emerged of greater proportions of older, female and older female patients. Gender differences were also apparent for different care groups within a total Community Health care population. Whilst females made much more use of services overall, in Community Health, older males were more likely to receive rehabilitative support services. Conclusion Age and gender profiles of health care agency and inter-agency populations clarify service use patterns and identify high proportions of women in health and social care populations, particularly in older care populations. This type of care population analysis could inform single agency and inter-agency shared care planning and commissioning.