There is a high prevalence of inactive adults in the UK, and many suffer from conditions such as cardiovascular disease (CVD) or poor mental health. These coexist more frequently in areas of higher socioeconomic deprivation. There is a need to test the effectiveness, acceptability and sustainability of physical activity programmes. Active Herts uses novel evidence-based behaviour change techniques to target physical inactivity. Active Herts is a community physical activity programme for inactive adults aged 16+ with one or more risk factors for CVD and/or a mild to moderate mental health condition. This evaluation will follow a mixed-methods longitudinal (baseline, and 3-month, 6-month and 12-month follow-ups) design. Pragmatic considerations mean delivery of the programme differs by locality. In two areas programme users will receive a behaviour change technique booklet, regular consultations, a booster phone call, motivational text messages and signposting to 12 weeks of exercise classes. In another two areas programme users will also receive 12 weeks of free tailored exercise classes, with optional exercise buddies available. An outcome evaluation will assess changes in physical activity as the primary outcome, and sporting participation, sitting, well-being, psychological capability and reflective motivation as secondary outcomes. A process evaluation will explore the views of stakeholders, delivery staff and programme leads. Economic evaluation will examine the programme costs against the benefits gained in terms of reduced risk of morbidity. This study was been approved by the Faculty of Medicine and Health Sciences Research Ethics Committee at the University of East Anglia. Informed written consent will be obtained from programme users in the evaluation. Results will be published in peer-reviewed journals, presented at conferences, and shared through the study website and local community outlets. ClinicalTrials.gov ID number: NCT03153098.