Thomas Orchard, 32, had been arrested on suspicion of a public order offence and taken to Heavitree Road police station in Exeter in October 2012.
He suffered a cardiac arrest whilst he was in custody and died in hospital seven days later.
At the trial, jurors heard that at one point, the officers held Orchard down and placed a large webbing belt over his face in order to restrain him.
He was then freed from restraints - making "little or no movement" - and then left lying face down in the locked cell for 12 minutes before officers re-entered the room to find that he was not breathing.
Custody sergeant Jan Kingshott, 45, and civilian detention officers Simon Tansley, 39, and Michael Marsden, 56, insisted their actions were proportionate and lawful.
The court heard that none of the defendants recognized that Orchard, a caretaker at St Thomas Church in Exeter, suffered from paranoid schizophrenia.
A jury at Bristol Crown Court found them not guilty of manslaughter after a six-week trial.
After the verdict, Chief Constable Shaun Sawyer said: "I fully recognise the impact of these long-running and difficult proceedings upon the family of Mr Thomas Orchard, his loved ones and friends, with whom my thoughts and condolences remain.
"Custody staff and colleagues within Devon and Cornwall continue to be professional and to serve our communities often under extreme and very difficult circumstances.
"Amongst our 25,000 or more detentions each year, so very many of those detainees are vulnerable through emotional crisis, mental ill health, physical ill health and substance misuse."