"Excessive force" was used by 11 police officers to restrain a man in custody four days before he died.
Olaseni Lewis, 23, had voluntarily admitted himself into Bethlem Royal Hospital, Beckenham, for mental health treatment on 31 August 2010.
But, medical staff called police when he became agitated. He died from his brain being starved of oxygen.
Delivering a narrative conclusion, an inquest jury found multiple failings by officers contributed to his death.
The jury said the use of restraint for two prolonged periods - 10 and 20 minutes - was "unnecessary and unreasonable".
It ruled: "The excessive force, pain compliance techniques and multiple mechanical restraints were disproportionate and unreasonable. On the balance of probability, this contributed to the cause of death."
Officers failed to follow training or take into account Mr Lewis's medical condition and they also failed in their training to "put an unresponsive person into the recovery position or administer basic life support".
South London and Maudsley NHS Foundation Trust (SLaM), which runs Bethlem, was also criticised by the jury.
"The admission process was unsatisfactory due to the lack of a full doctor's assessment, inadequate risk assessment and the failure to acknowledge the calming influence of family members," it said.
Failings in staff training were highlighted and a "lack of communication between medical staff and police throughout" was mentioned.
Mr Lewis passed out while being restrained and was transferred to hospital, where he died on 3 September.
Dr Matthew Patrick, chief executive of South London and Maudsley NHS Foundation Trust, said changes had been made since Mr Lewis's death.
"New processes have been put in place to improve how we train and support staff so that they can deliver safe care to people who become mentally unwell, and working alongside the police, we have improved how staff communicate and collaborate with them in high risk situations," he said.
Mr Lewis's parents, Aji and Conrad Lewis called for the Crown Prosecution Service to reconsider prosecuting the officers.
They also appealed for the new Met Police Commissioner, Cressida Dick, to meet them and assure them she understands what lessons needed to be learnt so "other families need not go through what we have had to endure".
An IPCC investigation into Mr Lewis's death concluded six officers had a case to answer for gross misconduct and five a case to answer for misconduct.
Three officers who had a case to answer for misconduct have since left the force and will face no further action, the IPCC said.
It added it would work with the Met Police to ensure disciplinary proceedings are held promptly.
Deputy Assistant Commissioner Richard Martin, responsible for professionalism, said how the force deals with mental health had changed "beyond recognition" in recent years.
"We focus on care rather than control and restraint must be considered as the last resort," he said.
"The MPS remains committed to on-going work around mental health with many initiatives introduced and improvements made in recent years."