Purpose: The purpose of this paper is to share New Zealand findings from the International Successful School Principalship Project (ISSPP) which relate to intrapersonal dimensions of leadership that promote principals' sustained success over time. Design/methodology/approach: Multi-site case study methods were used to describe the ongoing success of ten educational leaders, using ISSPP qualitative protocols for data gathering and cross-case inductive analysis. Findings: Findings identified the following influential intrapersonal factors that impacted positively on principals' leadership behaviours over time: their physical, mental and intellectual well-being; their levels of resiliency; and critical self-reflection. Practical implications: In light of these research findings, it is argued that these intrapersonal factors of a successful principalship are optimised when supported by an external agent. Originality/value: First, there are few research studies that have investigated how school principals maintain their success over time; second, principals traditionally receive mentoring or coaching support in their professional work but less so in their personal development. This paper advances a case for an external agent to help critique a principal's personal paradigm in order to promote well-being in the form of increased self-awareness and an understanding of why they think and act as they do.