Background Case formulation (CF) integrates information about an individual to conceptualise the factors causing and maintaining their current difficulties. It is a core clinical skill for both psychologists and psychiatrists. The recent UK government Offender Personality Disorder (PD) Strategy proposes a key role of probation staff in CF. Yet, CF in forensic settings has only recently received research attention, and it is unclear how and by whom it ought to be performed. Aims This study aimed to gain consensus on the key characteristics of CF in PD offenders. Methods We conducted an electronic Delphi survey of experts with a background in psychology or psychiatry. Results Consensus was achieved on who can perform CF, who it should be offered to and the benefits, process, theoretical models and content of CF. A cognitive–behavioural therapy or schema‐focused model was felt to be best suited to inform CF in PD offenders. No agreement was achieved on the assessment of quality of CF. Discussion Our findings support the government's PD strategy in that experts agreed that CF could be performed by professionals other than psychiatrists and psychologists. However, more research is needed on how to best assess the quality of CF and its utility in determining treatment pathways. Clinical implications It may be possible for probation staff to take on a more active role in CF for PD offenders using a cognitive–behavioural or schema‐focused model. However, a significant amount of training will be required for probation staff to acquire the relevant skills. Copyright © 2013 John Wiley & Sons, Ltd.