Abstract Background Cardiovascular disease (CVD) is the leading cause of premature death among people with serious mental illness (SMI). Sedentary behaviour (SB) is an independent risk factor for CVD and mortality and people with SMI are highly sedentary. We developed a health coaching intervention called ‘Walk this Way’ to reduce SB and increase physical activity (PA) in people with SMI and conducted a pilot randomised controlled trial (RCT) to test its feasibility and acceptability. Methods We randomised people with SMI from three community mental health teams into either the WTW intervention or treatment as usual. The WTW intervention lasted 17 weeks and included an initial education session, fortnightly coaching, provision of pedometers and access to a weekly walking group. Objective SB and PA were measured with accelerometers. Cardiometabolic risk factors and wellbeing measures were collected. Results We recruited 40 people of whom 33 (82.5%) were followed up. 13/20 (65%) of participants allocated to the coaching intervention completed it. In the intervention group SB decreased by 56 min and total PA increased by 32 min per day on average which was sustained 6 months later. There was no change in PA or SB in the control group. When interviewed, participants in the intervention found the intervention helpful and acceptable. No adverse events were reported from the intervention. Conclusions The intervention was feasible and acceptable to participants. Preliminary results were encouraging with improvement seen in both SB and PA. A larger study is needed to assess the effectiveness of the intervention and address any implementation challenges. Trial registration ISRCTN Registry identifier: ISRCTN37724980, retrospectively registered 25 September 2015.