In England and Wales, the Mental Capacity Act 2005 (MCA) provides a new legal framework to regulate substitute decision-making relating to the welfare of adults who lack the capacity to make one or more autonomous decisions about their care and support. Any substitute decision made on behalf of an adult lacking capacity must be in his/her 'best interests'. However, the value of adopting established principles and procedures for substitute decision-making in practice is uncertain, and little is known about the legal or ethical dynamics of social care support, including the day-to-day residential support provided to adults with intellectual disabilities (ID). Methods This paper reports a qualitative, grounded theory analysis of 21 interviews with support workers working in residential care homes for adults with ID, and observations of care practices. Results In contrast to the narrow legal responsibilities placed upon them, it is argued that support workers interpret substitute decision-making within a broad moral account of their care role, orientating their support towards helping residents to live 'a life like ours'. In so doing, support workers describe how they draw on their own values and life experiences to shape the substitute decisions that they make on behalf of residents. Conclusions Support workers' accounts reveal clear discrepancies between the legal regulation of substitute decision-making and the ways that these support workers make sense of their work. Such discrepancies have implications both for the implementation of the MCA, and for the role of support workers' values in the conceptualisation and delivery of 'good' care.