Mental health nurses will be posted at police stations across London from next month.
It is part of a pilot scheme to help forces assess vulnerable people and if necessary refer them for appropriate treatment.
If successful, the programme is expected to be rolled out across the country by 2017.
Policing Minister Damian Green said: "When someone is suffering a mental health crisis, the police are often the first to arrive on the scene.
"While the police cannot and should not provide the necessary medical support and treatment, they need to be able to recognise mental health issues and deal with them appropriately.
"The police also need their health partners to be ready to step in and provide medical support for people in crisis."