RationaleOnly half of newly implemented evidence‐based practices are sustained. Though poor sustainment can lead to negative consequences for clinical teams, organizations and patients, the causal explanations of sustainment are largely unknown.Aims and ObjectivesWe aimed to ascertain how (mechanisms) and in what circumstances (context) a newly implemented outcome measure in rehabilitation was sustained or not (outcome).MethodologyInformed by an integrated knowledge translation approach, we conducted a realist evaluation using a mixed method, embedded single case study design with data collection up to 18 months following the implementation of the Mayo‐Portland Adaptability Inventory ‐ version 4 (MPAI‐4), a rehabilitation outcome measure. Quantitative data (survey and patient charts) was analysed using descriptive statistics, then integrated with qualitative data (interviews with 10 key informants) and analysed using inductive and deductive retroduction. We integrated the data to develop a case description and ultimately, to refine the programme theory to better understand the sustainability of the MPAI‐4.ResultsWe linked context, mechanisms and outcomes, and also emphasized sustainability strategies in 18 explanations of how sustainability works. These explanations provide evidence for four overarching patterns: (1) implementation and sustainability phases are interconnected, (2) outcomes build on each other recursively, with patient benefits as the keystone outcome, (3) sustainment is achieved to varying levels across different sustainability outcomes [e.g., high level (e.g., rate of MPAI‐4 scoring: 77.7%) to low level (e.g., rate of MPAI‐4 application to clinical decision‐making: 3.7%)] and (4) the work of sustaining the MPAI‐4 is shared amongst different stakeholders.ConclusionImplementation teams can draw from this programme theory to improve the sustainment of outcomes measures while researchers could continue to refine the theory. Continued investigation of sustainability, including diverse and continuous sustainability outcomes, is needed to understand how to maintain improvements in quality of care and patient outcomes.