Ricky Hill walked out of Royal Derby Hospital where he was being treated for an overdose and hanged himself on site.
Coroner Dr Robert Hunter concluded there was no plan at the hospital to prevent him killing himself.
He told an inquest he would also write to Derbyshire police for a second time criticising its search procedures.
Dr Hunter returned a narrative conclusion at the inquest in Derby which found Mr Hill, 30, took his own life while an inpatient partly because "no effective measures were put in place to prevent him doing so".
Speaking after the hearing, his widow, Dawn Hill said: "I'm just glad that it's all over and we've got justice for Ricky because that's why we're all here."
The court heard Mr Hill walked out of hospital in September 2014 for a cigarette but never came back.
Security were alerted but took down an incorrect description which was then passed to police.
The first officer on the scene searched the site from a police car using the incorrect description.
Another officer later used an out of date satellite image from Google Earth to check the site, which still showed the hospital under construction.
Mr Hill's body was found by a local resident in a wooded area on hospital grounds in January 2015.
Although the coroner concluded he was dead before the police were called Dr Hunter still had "serious concerns" about the search and said he would write to the force's chief constable.
The court also heard nurses lacked training in dealing with mental health patients at the hospital and they had not been told about hospital guidelines.
Dr Hunter said he would write to the Department of Health raising concerns that changes should be made at all hospitals.
Following the conclusion of the hearing Deputy Ch Con Peter Goodman said: "We want to apologise to the family of Ricky Hill.
"We recognise we made mistakes at the time he went missing and subsequent in our search for him there were areas we could have done better."
He added the force would work with the coroner to address further concerns.
Cathy Winfield, chief nurse at the Royal Derby Hospital, also apologised to the family during the hearing and offered her condolences.
Speaking afterwards, she said: "We've run a robust teaching programme for doctors and nurses to help them identify [mental health needs] but also assess the level of risk patients might be facing."
A coroner has criticised police and medics for a "catalogue of errors" after a man killed himself and was not found for three months.