The 28-year-old actress arrived at the Las Encinas Hospital for a three-day stay after arriving in Los Angeles from New York late last week, Arminak added.
A so-called 5150 hold is used on those deemed to have a mental illness that makes them a danger to themselves or others. She was placed under the same hold last July after allegedly starting a fire in the driveway of a home in Thousand Oaks, California.
The hold can be extended if a judge deems it necessary.
Bynes' parents first expressed concern on Friday, the same day their daughter went on a troublesome Twitter rant.
"My clients are very concerned about their daughter," Arminak told ABC News in a statement. "Despite what is being reported, they are doing everything they can to help Amanda."
Bynes spent the week in New York after flying in last week from California, where she was arrested for a DUI on Sept. 29, according to authorities.
The California Highway Patrol confirmed that after the actress stopped in the middle of an intersection, she was pulled over and an officer determined that she appeared to be under the influence. After she failed a field sobriety test, Bynes was arrested, police said.
Bynes is due in court on Oct. 23 and expected to enter a plea on the DUI.
After her 5150 hold last summer, Bynes entered rehab and her parents got a conservatorship over the actress after her treatment.
She left rehab in December and, shortly thereafter, enrolled in fashion school. She has since said she is no longer in school.