Dina Lohan confirmed to E! that her daughter has left rehab, calling it "a great day."
Lindsay Lohan made a hopeful post on Twitter, writing, "Today is the first day of the rest of my life."
She then quoted Mahatma Gandhi, posting, "The future depends on what we do in the present."
Judge Elden Fox sentenced Lohan, the "Mean Girls" star, to her latest stint in rehab in October after she failed a drug test.
Dr. Mitch Rosenthal, a psychologist and the founder of Phoenix House, a substance abuse service organization, said Lohan must make her recovery paramount.
"She's got to surround herself with sober friends and work very hard at maintaining sobriety. It isn't a part-time thing," Rosenthal said. "It is really saying, 'I have a very serious illness and I have to work at it the way I have to work at hypertension and diabetes, I have to manage this illness.'"
As a child-actress who grew up on screen and in the public eye, Lohan saw the Hollywood nightlife begin to take a toll on her in 2007.
She was arrested twice. She served three jail sentences and five rehab stints. She pleaded guilty in August 2007 to two misdemeanor counts of being under the influence of cocaine, no contest to two counts of driving with a blood-alcohol level above 0.08 percent and one count of reckless driving.
The first time Lohan was sent to prison she broke down, sobbing in the courtroom. After being released, her ankle bracelet monitor revealed that she had slipped again.
Lohan poked fun of herself at the MTV Video Music Awards in September, telling host Chelsea Handler, "Don't you know nobody wants to work with a drunk, take it from me ... get a hold of yourself."
Her latest stint in rehab had been relatively quiet until last month when police were called to the Betty Ford Center early Dec. 12 after a staffer accused Lohan of battery.
"On Dec. 12, 2010 at 1:03 a.m. officers from the Palm Desert Police Department conducted a battery investigation," the statement said. "The incident involved Lindsay Lohan and another female adult who is a staff member. Lohan was reported as a suspect in the investigation. The victim desired prosecution."
The staff member was fired but Lohan remains under investigation.
Rosenthal said Lohan's sobriety will depend, in part, on with whom she surrounds herself.
"She's got to be surrounded with sober people," he said. "She's got to stay out of places that give her cues for drug use and drinking. She should not be in bars. She should not be at friend's homes where liquor is being used or marijuana is being used."
Rosenthal said rehab might have given Lohan clarity on her relationship with her father.
"It would be great if they [Michael and Lindsay] worked on it [sobriety] together and it would be great if he is a healthy influence on her life. She's going to have to decide that," Rosenthal said.