Her mother Lynn Bynes was named as the conservator over her daughter's estate, while Amanda's dad Rick named as an interested party.
The conservatorship, a legal concept in US, where one is subjected to the legal control of an external entity or organization, will remain in place until Sep 30, when a judge will decide if the arrangement will be made permanent, reports tmz.com.
During the court's proceedings, which Amanda did not attend, the judge also approved doctors' request to keep her under medical supervision for another 30 days, despite opposition from her lawyer.
The "She's The Man" star was placed on an involuntary psychiatric hold last month after she started a small fire on a driveway in her parents' neighbourhood in California.
The court development is the latest in a string of woes for the actress. She is facing a charge of driving under the influence after allegedly crashing her car into a police vehicle in 2012, and a further accusation of driving on a suspended license just months later.