The experiences of mothers of adult offspring with Angelman, Cornelia de Lange, or Cri du Chat syndrome have not been previously explored in research. The current study focuses on experiences with social and medical services and the impact the rareness of an adult offspring's syndrome has on the experiences of mothers. Eight mothers of adults with Angelman, Cornelia de Lange, or Cri du Chat syndrome were interviewed. Thematic Content Analysis was used to interpret the interviews. Four themes emerged from the analysis: (i) The rarity of their offspring's syndrome, (ii) Uneven medical and social care service provision, (iii) The inertia of social care services, and (iv) Mothers as advocates. Mothers felt that the rareness of their offspring's syndrome did not affect experiences with social care services, but did affect access to medical services and some aspects of day-to-day living. Accessing appropriate social care services was reported to be a lengthy and complex process. These data may help inform care service providers about how best to support adults with rare genetic syndromes and their families.