A growing number of informal caregivers of older adults combine caregiving with a paid job, raising the question of whether they will be able to meet the increasing demand for informal care. The aims of our study are twofold: first, to describe the development of a model providing insight into the factors that play a role in the availability and provision of informal care by working caregivers of older adults, and second, to investigate which societal developments will impact the factors in the model, and thus the future availability of informal care by workers. A mixed-methods approach was applied to integrate evidence from academic and grey literature, with insights from experts through a Group Model Building exercise and a Delphi study. The resulting Working Informal Caregiver (WIC) model presents a range of individual, social and environmental factors that are related to working caregivers’ ability and their willingness to engage in informal care. Experts foresee that future informal care will be impacted most by the increasing participation of women in the workforce, while changing household structures may diminish opportunities to share care tasks within their households or families. The WIC model can be used to gain better insight into the availability of informal care by workers, now and in the future.