The 24-year-old "Mean Girls" and "Freaky Friday" star underwent court-ordered treatment at the rehab center after violating probation several times for a 2007 driving under the influence conviction.
"Today is the first day of the rest of my life," ABC News quoted Lohan as saying via Twitter.
"The future depends on what we do in the present," she said, quoting Mahatma Gandhi.
E! News said it was unclear from its sources where Lohan was headed when she left the treatment center early Monday.
She is due back in court for a progress hearing Feb. 25.