Employees with mental health conditions often struggle to remain in employment. During the COVID-19 pandemic, these employees faced additional stressors, including worsening mental health and work productivity. In 2020, as part of a larger programme of work called the Mental Health and Productivity Pilot (MHPP), we developed a new early intervention (MENTOR) that jointly involved employees, managers, and a new professional (Mental Health Employment Liaison Worker, MHELW). The intervention involved trained MHELWs delivering ten sessions to employees with existing mental health conditions and managers (three individual sessions and four joint sessions) over twelve weeks. These sessions aimed to improve psychological flexibility, interpersonal relationships, and engagement of employees. This feasibility randomised controlled trial aimed to examine the feasibility and acceptability of the intervention from the perspective of employees and managers using a mixed methods approach. The intervention was largely considered feasible and acceptable. Initial findings suggest there may be benefits for employees productivity, mental health, and managers' mental health knowledge. Logistical challenges acted as a barrier to the participation of employees and managers in the trial and their retention throughout its duration. The major strengths of this study were the co-design and inter-disciplinary approach taken. Overall, findings suggest that this novel intervention has potential but needs some adjustments and testing in a larger sample. •We developed an early intervention (MENTOR) to support employees with mental health conditions to help them stay engaged and productive at work.•MENTOR involves individual and joint sessions with employees and their managers, delivered by Mental Health Employment Liaison Worker (MHELW).•The intervention aims to improve psychological flexibility, interpersonal relationships and productivity.•The co-design and interdisciplinary approach were major strengths of the study, suggesting potential for future research and application.