Abstract This paper reports findings from interviews with practicing public librarians in Aotearoa New Zealand. It details respondent understanding of influences on perception and behaviour, and critical incidents as they relate to librarian professional identity. In-depth semi-structed interviews were conducted with forty practicing public librarians. Interviews were analysed with an inductive approach. Findings report on sample tendencies of dominant influences on practitioners’ perception and behaviour, impacting professional identity development over time. Dominant influences are respondent understanding of the strongest aspect they understand to influence their professional identity for both their individual perception of their professional identity and social factors influencing their professional behaviour. Findings also report on the identity negotiation process prompted by critical incidents, whose criticality is reliant on individual perception of incident in relation to professional identity. This process outlines affective response to the critical incident as a gateway to identity negotiations, leading to a discovery and/or growth of an identity facet, which will either affirm or undermine identity understanding. This process may be iterative, as meaning ascribed to the incident may change over time. Three theoretical propositions are presented articulating the role of dominant influences and critical incidents on identity negotiations of public librarian professional identity.