The "deplorable" conditions violated a ban on cruel and unusual punishment in the US Constitution, its report said.
Jail officials, it said, had failed to address a "dramatic increase" in suicides over the past two years.
But the sheriff's office said it was "disappointed" the report "fails to fully recognise" improvements made.
"The report also mischaracterises and significantly understates the incredible efforts made to improve our suicide prevention practices," the sheriff's office said in statement.
The US justice department said the nation's largest jail system - with about 19,000 prisoners - had completed some of the reforms required by a 2002 court agreement.
It also applauded steps to expand community supervision programmes, saying a rapid increase in the number of prisoners who are seriously mentally ill was putting the system under strain.
But it said "serious systemic deficiencies" and "deplorable environmental conditions" remained for those being held at the jail, violating the US constitutional ban on cruel and unusual punishment.
Among the criticisms in the report was a lack of response to an increase in suicides and suicide attempts at county jails.
"Fifteen suicides in 25 months produced almost no discernible change in the jails' custodial practices," the report said.
Other findings include:
The sheriff's office said it remains committed to working with the Justice Department on the issues.
Jails in the US are generally for people being locked up in the short-term, but the prison system throughout the state of California, where inmates serve longer sentences, has also come under scrutiny for overcrowding.

Jordan Burrow tripped James Jennings in the Town box, and Parkin put the visitors ahead from the rebound after his penalty was saved by Sam Johnson.
Frear made it 2-0 before the break when he fired in Keanu Marsh-Brown's cross.
Darren Carter's shot hit the bar for Rovers in the second half, while Halifax struggled to create chances.
Forest Green assistant Jamie Day told BBC Radio Gloucestershire:
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"They were on a good run of games - 13 unbeaten - so we knew it was going to be a tough match, but I thought the boys dug in today and showed quality when needed and fully deserved the win.
"The way Cheltenham are playing as well, we need to keep winning games.
"We've had two really good runs during the course of the season and we want to keep it going."

23 May 2016 Last updated at 18:35 BST
He's been given the boot after two years.
United finished fifth in the Premier League this season, which means missing out on the Champions League.
Winning the FA Cup on Saturday wasn't enough to save van Gaal.
Former Chelsea boss Jose Mourinho is now expected to take over at Old Trafford.

The official data prompted the government to upgrade growth forecasts for the second time in a week and to promise no new austerity measures.
Last week, growth for 2014 was up-rated to 3% from 2.1%, but will now be around 4.5%, Ireland's finance minister said.
Michael Noonan said the economy was in "a catch-up phase... after recession".
The year-on-year growth of 7.7% was the strongest recorded in Ireland since the early 2000s, Mr Noonan told reporters. By comparison, the eurozone recorded growth of just 0.7% over the same period.
Along with Greece, Spain and Portugal, Ireland was among the eurozone's worst hit economies, with the bursting of its property bubble and soaring unemployment.
The 1.5% quarterly rise beat the 0.5% expected by economists polled in a survey by the Reuters news agency.
Mr Noonan said the faster growth would mean Ireland's budget deficit falling to about 3.5% of GDP this year against a target of 4.8%. That meant 2bn euros of tax rises and spending cuts planned for next year would no longer be needed to reach the EU limit of 3% by the end of 2015, he said.
The economic improvement comes after years of austerity, a bailout from international lenders, and relentless austerity measures.
"At face value, the numbers suggest that the boom is back," said Austin Hughes, chief economist at KBC Bank Ireland.
"It shows a clearly improving economy. It's very encouraging to see the improvement in consumption, in construction. So a domestic recovery is taking hold, if a lot less intense that the headline number would suggest," he said.
The condition of mental healthcare inside Los Angeles County jails is so poor that it is unconstitutional, the US justice department has said.
[NEXT_CONCEPT]
Goals from Jon Parkin and Elliott Frear helped promotion-chasing Forest Green Rovers end Halifax Town's 13-match unbeaten run in the National League.
[NEXT_CONCEPT]
Manchester United fans have been reacting to manager Louis van Gaal's sacking from the club.
[NEXT_CONCEPT]
The Republic of Ireland's economy grew 1.5% in the second quarter of the year, figures show, and was up 7.7% on the April-to-June period in 2013.