Bynes, 27, allegedly caused a disturbance when she tried to break into a retirement community in her hometown of Thousand Oaks Sunday.
The following night, she was involved in an altercation in the same area, which led to police involvement and her eventual hospitalization.
The celebrity news website said it is unclear exactly what Bynes did Monday, but some sources said she tried to set a fire in someone's yard.
"Deputies investigated the incident and determined that she met the criteria of 5150 W&I [California Welfare & Institutions Code.] She was detained and taken for a mental health evaluation,"
Capt. Don Aguilar of the Ventura County Sheriff's Office told Hollyscoop.
The report did not say where Bynes is undergoing evaluation and treatment.
The actress is known for her work on "The Amanda Show," as well as in the films "Hairspray" and "What a Girl Wants," but her last major screen credit was a supporting role in 2010's "Easy A."
Her once-bright career has been eclipsed in recent years by her bizarre Twitter rants and media reports of her erratic behavior and brushes with the law.