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.
The condition of mental healthcare inside Los Angeles County jails is so poor that it is unconstitutional, the US justice department has said.