The dad-of-two, of Ashburnham Road, Luton, died on November 4, 2013, after he was detained in the street and held in police custody.
A jury was sworn in for his inquest at Milton Keynes and reached a conclusion that his death was ‘contributed to by neglect’ on Friday.
The jury criticised the restraint used by Beds Police officers, as well as paramedics from East of England Ambulance Service, who failed to assess Mr Briggs at the scene.
Mr Briggs was held face down for over 13 minutes, and in handcuffs and leg restraints for 25 minutes.
Deputy Chief Constable Trevor Rodenhurst said: “The family and friends of Leon Briggs have had to wait far too long to hear all the facts surrounding his death and our thoughts are of course with them at this very difficult time.
“The jury has today identified a number of significant failings by the police which contributed to the death of Mr Briggs and for this we are truly sorry."
On the day of his death, Mr Briggs was spotted moving erratically in the street. Paramedics attended but did not communicate with him, while he was detained by police officers under the Mental Health Act and taken to Luton Police Station.
After being found unresponsive in his cell, Mr Briggs was taken to Luton & Dunstable Hospital where he was pronounced dead.
DCC Rodenhurst said: "This inquest focused on a period of 36 minutes as we responded to public concerns for Mr Briggs who was suffering a drug-induced psychosis triggered by amphetamine levels described by one expert as being ‘off the scale’.
“Unbeknown to officers he also had a serious underlying heart condition. There is no easy way of managing such a situation."
The tragic incident made national headlines and an investigation was carried out by former police watchdog The Independent Police Complaints Commission, which referred its findings to the Crown Prosecution Service in 2014.
The then-serving Beds Police and Crime Commissioner (PCC) Olly Martins was temporarily suspended by the Labour Party that same year after disclosing details of the IPCC investigation.
In September 2018, the Crown Prosecution Service decided that no action would be taken against the five officers and one other police staff member involved in Mr Briggs' detention - a decision which "devastated" his mother Margaret.
A misconduct hearing by police watchdog The Independent Office for Police Conduct (IOPC) then collapsed in February last year, with the IOPC and Beds Police blaming each other for the failed hearing - described as a "shambles".
The IOPC claimed Beds Police had not fully co-operated with them, while the latter complained that former watchdog the IPCC had led a flawed investigation, which had been criticised in an internal review.
DCC Rodenhurst added: “We’ve made extensive changes since 2013 but we remain absolutely committed to working with the coroner and all of our partners in order to identify and make any necessary further improvements."
The officers themselves have made no comment, but they were represented by the Bedfordshire branch of the Police Federation - which issued a defiant statement to the media after the inquest.
Bedfordshire Police Federation secretary Emma Carter said: "Six colleagues were suspended from duty for more than five years as part of this process. A cloud has unfairly been hanging over them since 2013.
"This has changed their lives; their family's lives and the officer's careers immeasurably.
"This now thankfully concludes today, and we hope this also brings some long-awaited answers for Leon's family.
"We must pay credit to our colleagues for maintaining their professionalism and dignity throughout the process - a CPS investigation, a police complaints process and now an inquest - and we wish them and their families well as they get on with their lives."