Judge Brenda Penny, who on Wednesday was to receive a review of the conservatorship arrangement, which has been in place for 11 years, ruled that because medical, mental health and child issues will be discussed, the courtroom should be sealed.
Spears was not in court initially, but in previous hearings, she has entered through back doors after the courtroom was cleared.
Her parents were in attendance. Her dad, Jamie, 67, temporarily stepped down earlier this month from his role as conservator over her personal life, though he maintained financial control over her.
On Tuesday, prosecutors declined to file charges against Jamie Spears after a child-abuse investigation.
Montgomery will reportedly fill the role until Jan. 31, 2020, at which point, it's speculated that Jamie will try to renew the role.
The site claims that Montgomery will have the same powers as Jamie: She can limit or restrict Britney's visitors, retain security and caretakers for Britney, enforce restraining orders (including the ones the Spears family has against the singer's former manager, Osama "Sam" Lutfi), and communicate with medical personnel regarding her care and history.
The news comes just after TMZ reported that the psychiatrist in the conservatorship case, Dr. Timothy Benson, died suddenly on Aug. 24.
Spears' ex-husband and father of her two sons, Kevin Federline, obtained a restraining order against Jamie on behalf of himself, Sean and Jayden James.
According to a law enforcement source, Sean allegedly “felt unsafe” and locked himself in a room after an argument with Jamie, but Jamie managed to break in and allegedly “shook” the boy.
Britney reportedly removed her sons from her father's home and informed Kevin of the incident. People reported that Kevin now has 90 percent custody of Jayden and Sean to their mom's 10 percent. They previously shared 50-50 custody.
Fox News' Jessica Sager and The Associated Press contributed to this report.