Whiplash-associated disorder (WAD) causes substantial social and economic burden, with ≥70% patients classified as WADII (neck complaint and musculoskeletal sign(s)). Effective management in the acute stage is required to prevent development of chronicity; an issue for 60% of patients. An Active Behavioural Physiotherapy Intervention (ABPI) was developed to address both physical and psychological components of WAD. The ABPI is a novel complex intervention designed through a rigorous sequential multiphase project to prevent transition of acute WAD to chronicity. An external pilot and feasibility cluster randomised double-blind (assessor, participants) parallel two-arm clinical trial was conducted in the UK private sector. The trial compared ABPI versus standard physiotherapy to evaluate trial procedures and feasibility of the ABPI for managing acute WADII in preparation for a future definitive trial. Six private physiotherapy clinics were recruited and cluster randomised using a computer-generated randomisation sequence. Twenty-eight (20 ABPI, 8 standard physiotherapy) participants [median age 38.00 (IQR = 21.50) years] were recruited. Data were analysed descriptively with a priori establishment of success criteria. Ninety-five percent of participants in the ABPI arm fully recovered (Neck Disability Index ≤4, compared to 17% in the standard physiotherapy arm); required fewer treatment sessions; and demonstrated greater improvement in all outcome measures (pain intensity, Cervical Range of Motion, Pressure Pain Threshold, EuroQol-5 Dimensions) except for the Impact of Events Scale and Fear Avoidance Beliefs Questionnaire. The findings support the potential value of the ABPI, and that an adequately powered definitive trial to evaluate effectiveness (clinical, cost) is feasible with minor modifications to procedures.