The Health Inequalities Assessment Toolkit (HIAT) was developed to support those involved in health research to integrate a focus on health inequalities. Our study focuses on bringing together the concepts of boundary objects (BO) and brokers-as-bricoleurs to explain the implementation of the HIAT within a research capacity building programme. Exploring the extent to which (i) the HIAT operated as a BO and (ii) the ideal conditions to nurture and enhance its effectiveness during knowledge mobilisation. We employed a qualitative approach to analyse: semi-structured focus groups and telephone interviews; secondary data from an evaluation of the wider research programme within which the capacity building was situated. Data was thematically analysed incorporating the properties of a BO: meaningfulness, convergence, resonance and authenticity. Four main themes identified: (1) Generating convergence through creating a focus (2) Reconciling differences to create a common language (3) Workshop facilitators: boundary brokers-as-bricoleurs, (4) Thoughts into action. The HIAT operated as a BO, enabling individuals across the different project teams to galvanise around the issue of health inequalities, explore collaboratively and incorporate equity within service evaluations. Highlighting the importance of involving brokers with an ability to improvise and mobilise around the HIAT, using their expertise to translate and interpret across boundaries and emphasise shared goals. Reflecting on this, a modified tool with additional resources beyond socio-economic causes has been launched as a forum to consider health inequalities from diverse perspectives for use beyond UK health and social care research.