Purpose Mental health concerns are common after a workplace injury, particularly amongst those making a compensation claim. Yet there is a lack of research exploring the effect of modifiable elements of the return-to-work process on mental health. The aim of this study is to examine the impact of perceived injustice in the interactions between claim agents and claimants on mental health symptoms in the 12-month following a musculoskeletal (MSK) workplace injury. Methods A cohort of 585 workers compensation claimants in Victoria, Australia were interviewed three times over a 12-month period following a workplace MSK injury. Perceptions of informational and interpersonal justice in claim agent interactions were measured at baseline, and the Kessler Psychological Distress (K6) scale was administered as a measure of mental health at all three timepoints. Path analyses were performed to examine the direct and indirect effects of perceived justice at baseline on concurrent and future mental health, after accounting for confounding variables. Results Each 1-unit increase in perceptions of informational and interpersonal justice, indicating poorer experiences, was associated with an absolute increase of 0.16 and 0.18 in respective K6 mental health score at baseline, indicating poorer mental health on a 5-point scale. In addition, perceived justice indirectly impacted mental health at 6-month and 12-month, through sustained negative impact from baseline as well as increased risk of disagreements between the claim agent and claimant. Conclusions This finding has highlighted the importance of perceived justice in claim agent interactions with claimants in relation to mental health following a work-related MSK injury.