Background: Interactions between individuals experiencing mental health (MH) problems and the police are complex, and effectiveness of innovative support and diversion models in England and Wales not yet fully evaluated. Aims/hypotheses: Our aims were to examine police interactions with suspects and to measure the immediate effectiveness of police/NHS MH interventions, including liaison and diversion and embedded staff in police contact and control rooms. We hypothesised that those with an MH flag would have significantly greater benefit from such interventions than those without in terms of how far they are taken down the criminal justice pathway and how long they spend in police custody. Methods: We examined police interactions with suspects with and without flagged MH problems in relation to key outcome measures over a 15-month period, overall or when flagged by nurses alone. ‘MH flagging’ is defined as the presence of a marker on police systems, including both historical and current information, that alerts control room staff and response officers that the call may involve an individual/s with MH problems. Serial cross-sectional analysis of material from a database of individual cases integrating information from three police sources (N = 13,472) was used to test for associations between ‘mental health flagging’ and outcomes. Results: Individuals with an MH flag have almost identical police dispatch response profiles to those without; they were arrested for and charged with similar offences. Those with an MH flag were significantly more likely to be charged with a criminal offence, less likely to receive a caution and spent longer periods in police custody than people under similar accusations but no MH flag. Conclusions: MH flagging appeared to disadvantage the people flagged, despite the presence of theoretically appropriate interventions. Further research is needed to understand this. It may be that indicating this form of vulnerability if the person is not judged to qualify for a MH service is discriminatory and may even account for excessive rates of mental disorder among prisoners.