According to People magazine, the 24-year-old was taken to the Ronald Reagan UCLA Medical Center in Westwood to begin her three-month stay in a treatment center for drugs and alcohol.
A black SUV was seen arriving at the hospital early Monday, but it was unknown if Lohan or family members were in the vehicle.
A caravan of SUVs were also seen leaving the jail in Lynwood, but it was unconfirmed if Lohan was in any of those vehicles.
Media camped outside the jail hoping to catch a glimpse of the actress, but unlike Paris Hilton's conspicuous exit, Lohan chose a quieter route and left unnoticed through a side entrance.
According to Steve Whitmore of the L.A. County Sheriff's Department, Lohan was released at 1:35 a.m. and sent directly to a treatment center. She had been in jail since July 20 after violating her probation in two DUI cases.
"She will now be under the supervision of the L.A. County probation department," Whitmore said.
Lohan was hoping to get 24 hours with her family before going into rehab, but her request was not granted. It was also expected that Lohan would go to a rehab center in Newport Beach or Costa Mesa, but according to People magazine, the judge deemed those facilities not secure enough for the actress.
Lohan could be sent back to jail if she violates the rules. When she gets out of rehab, she'll have to undergo random drug and alcohol testing for 12 months.