Ventura County Superior Court Judge Glen Reiser granted a temporary conservatorship motion brought by Bynes' parents, the documents said. They filed the request after the 27-year-old actress allegedly started a small fire in front of a home in the Los Angeles suburb of Thousand Oaks last month.
Ventura County Sheriff deputies put the former teenage star of Nickelodeon series All That in involuntary psychiatric care at a local hospital after the July 22 incident, and Bynes has been there ever since.
Bynes' mother, Lynn Bynes, will now have control over her financial and legal affairs. She would also gain control over Bynes' medical treatment if the actress is released from court-ordered treatment.
It is rare for a judge to grant conservatorship in the case of adults. Most famously, pop singer Britney Spears was put under conservatorship in 2008 following a personal meltdown and divorce. Her father and lawyer remain in control of her affairs, even though her singing career has rebounded.
Bynes was born and raised in Thousand Oaks. She starred in the films What A Girl Wants, She's The Man and Hairspray but has made headlines over the past year for erratic behaviour, legal problems and odd posts on Twitter.
Bynes, who is on probation in California for driving on a suspended licence, faces charges in the state for allegedly driving under the influence and in New York for marijuana possession and throwing a glass bong out of her 36th-floor Manhattan apartment.