Purpose - to investigate the phenomenon of sustainable social (enterprise) entrepreneurship from both organisational and individual/personal identity perspectives. Two research questions ask: (RQ1) what are the key organisational identity (OI) and governance issues associated with sustainable social enterprises (SEs) and social entrepreneurship?, and; (RQ2) who are the social enterprise (SE) leaders/entrepreneurs (and why are they important from an identity perspective)? Design/methodology/approach – A stage 1 interpretative phenomenological analysis (IPA) employed 30 semi-structured interviews of social entrepreneurs in the (UK) South East; a macro-level conceptualisation of social enterprise (organisational) identity, sustainability and governance issues being the key deliverable. This was followed by a stage 2 social constructionist and social entrepreneurial identity study; whereby, 16 in-depth interviews, with 3 case study social enterprise leaders were conducted over a period of three years. Stage 3 involved 2 employee and volunteer focus groups to help triangulate data from the previous stages. Finally, stage 4 follow-up interviews with 4 selected informants helped evaluate the impact of the Brexit vote on my thesis arguments. Various third sector and government policy documents were consulted throughout the study. Findings – Firstly, I argue that understanding who organisations are, as well as, what they do, are important for understanding the sustainability of social enterprises, and the third sector. A new conceptual social enterprise grid (SEG) is developed to distinguish who social enterprises are; relative to other third and public-sector organisations. Secondly, a Ricoeurian narrative analysis helps demonstrate the agentic role of social entrepreneurs; how social enterprise sustainability is motivated by personal beliefs, social values and an idem (i.e. almost permanent) sense of identity. Similarly, results demonstrate how social enterprise sustainability could be at least part-attributed, to the lifetime agentic function of social entrepreneurs. Originality/value – This PhD thesis addresses fundamental definitional and theory gaps in the social enterprise and third sector identity literatures. It contributes by offering fresh perspectives on the complex and inter-related issues of (organisational and socio-entrepreneurial) identity, governance and sustainability.