Increasing physical activity in the workplace can provide physical and mental health benefits for employees and economic benefits through reduced absenteeism and increased productivity for the employer. However, there is limited evidence on effective behaviour change interventions in workplace settings that lead to maintained physical activity. This study aimed to address this gap and contribute to the evidence base for effective, and cost-effective, workplace interventions. To determine the effectiveness and cost-effectiveness of the Physical Activity Loyalty (PAL) Scheme, a multi-component intervention based on concepts similar to those that underpin a high-street loyalty card, aimed at encouraging habitual physical activity behaviour and maintained increases in mean steps/day. A cluster-randomised controlled trial and embedded economic evaluation, behavioural economic experiments, mediation analyses and process evaluation. Office-based employees from public sector organisations in Belfast and Lisburn city centres, Northern Ireland. 853 participants (mean age 43.6 years (SD 9.6); 71% female) were randomly allocated by cluster to either the Intervention Group or (Waiting-List) Control Group. The six month intervention consisted of financial incentives (retail vouchers), feedback and other evidence-based behaviour change techniques. Sensors situated in the vicinity of the workplaces allowed participants to monitor their accumulated minutes of physical activity. The primary outcome was mean steps/day recorded using a sealed pedometer (Yamax Digiwalker CW-701, Japan) worn on the waist for seven consecutive days at six and 12 months post-intervention. Secondary outcomes included health, mental wellbeing, quality of life, work absenteeism and presenteeism and use of healthcare resources. The mean steps/day were significantly lower for the Intervention Group compared to the control group (6,990 [SD 3,078] vs 7,576 [SD 3,345] respectively, adjusted mean difference=-336, 95% CI:-612 to-60, P= 0.02) at six months post-baseline, but not significantly lower at 12 months post-baseline. There was a small but significant enhancement of mental wellbeing, in the Intervention Group (difference between groups for the Warwick-Edinburgh Mental Wellbeing Scale [WEMWBS] of 1.34 points, 95% CI: 0.48 to 2.20), but not for the other secondary outcomes. An economic evaluation suggested that, overall, the scheme was not cost-effective compared to no intervention. The intervention was£ 25.85 (95% CI:-29.89, 81.60) more costly per participant but had no effect on QALYs (incremental QALY=-0.0000891, 95% CI:-0.008, 0.008). Significant re-structuring of participating organisations during the study resulted in lower than anticipated recruitment and retention rates. Technical issues affected intervention fidelity. Overall, assignment to the Intervention Group resulted in a small but significant decline in mean pedometer-measured steps/day at six months relative to baseline, compared to the waiting-list control group. The PAL Scheme was deemed to be not cost-effective compared to no intervention, primarily due to no additional QALY gained through the intervention. Research to better understand the mechanisms of physical activity behaviour change maintenance will help the design of future interventions.