In this paper, we explore how the use of a specific mentoring model focusing on the evolution of the relationship between mentor and mentee, may influence the incidence of failure. In our research we employed a case study methodology to examine a regional public service mentoring scheme in the UK where a developmental relationship mentoring model had been developed and used to guide practice. Findings indicated toxicity and negative outcomes may be positively influenced by mentor motivation and emotional intelligence, and can be avoided when there is awareness of how relationships develop and evolve. For example, the use of contracting in the early stages can limit the mismatched expectations that provoke disappointment, but equally mentor actions at other stages play key roles in reducing potential failure. Our study has implications for the enhancement of mentor training and scheme coordination as well as contributing to the understanding of negative mentoring relationships.