COVID-19 was identified as a pandemic by the World Health Organisation (WHO) in December 2020. Advanced Clinical Practitioners (ACPs) in England working with older people with frailty, experienced their clinical role changing in response to the emergency health needs of this complex population group. In contrast to other countries, in England Advanced Clinical Practitioners are drawn from both nursing and allied health professions. Whilst much of the literature emphasises the importance of ensuring the sustainability of the Advanced Clinical Practitioners' role, the pandemic threw further light on its potential and challenges. However, an initial review of the literature highlighted a lack of research of Advanced Clinical Practitioners' capabilities working with uncertainty in disaster response situations. To capture the lived experience of how English Advanced Clinical Practitioners working with older people adapted their roles in response to the COVID-19 pandemic (October 2020-January 2021). A qualitative research design was used. Following ethical approval, 23 Advanced Clinical Practitioner volunteer participants from across England with varied health professional backgrounds were recruited from Advanced Clinical Practitioners' professional and social media networks on Twitter using a snowballing technique. Depending on preference or availability, 23 participants (nurses (18), physiotherapists (2), paramedics (2) and a pharmacist (1)) were interviewed singularly (n = 9) or as part of 3 focus groups (n = 14) using Zoom video communication. Audio recordings were transcribed and using qualitative data analysis software, NVivo 12 pro, coded for an essentialist thematic analysis of Advanced Clinical Practitioners' responses using an inductive approach. 27 codes were identified and collated into five themes. For the purposes of this paper, four themes are discussed: experiencing different work, developing attributes, negotiating barriers and changing future provision. Advanced Clinical Practitioners successfully transferred their advanced practice skills into areas of clinical need during the pandemic. Their autonomous and generic, high level of expertise equipped them for management and leadership positions where speed of change, and the dissolution of traditional professional boundaries, were prioritised. Barriers to progress included a lack of knowledge of the Advanced Clinical Practitioner role and friction between Advanced Clinical Practitioners and physicians. The study demonstrated the successful adaption of the Advanced Clinical Practitioner role to enable more creative, personalised and sustainable solutions in the care of older people living with frailty during the pandemic. The potential of Advanced Clinical Practitioner development is in a juxtaposition to the threat of pandemic services being dismantled once the emergency nature of care has passed. Healthcare organisations have a vital part to play in considering the enablers and barriers of Advanced Clinical Practitioner capability-based practice when responding to uncertainty. Copyright © 2022 Elsevier Ltd. All rights reserved.