Browning wept in court as prosecutors described the crime. A sheriff's deputy brought him a box of tissues.
In exchange for the plea, prosecutors will not seek a sentence of life without parole. Instead, they will seek a maximum of two consecutive and two concurrent life sentences, meaning Browning could eventually be released on parole.
He was a week shy of his 16th birthday at the time of the slayings, too young under state law to face the death penalty.
According to police, Browning walked home after midnight from a friend's house and shot his parents and brothers in the heads as they slept, then returned to the friend's house and pretended nothing had happened. Browning later confessed to the shootings and told police where they could find the murder weapon.
Browning's lawyers were not successful in their attempt to transfer the case to juvenile court.
In a hearing in July, Dr. Neil H. Blumberg, a forensic psychiatrist, testified that Browning suffered from a dissociative disorder on the night of the killings with disruptions in consciousness, memory and perception. He said Browning indicated he was in "a trancelike state."
Blumberg testified that relatives and friends told him they had seen the parents abuse Nicholas. He said the family had a history of alcoholism and that John, Tamara and Nicholas all abused alcohol.
But Baltimore County Circuit Judge Thomas Bollinger ruled this summer that evidence about Browning's mental condition alone was not sufficient to merit transferring the case to juvenile court.
On Monday, Browning stood between his attorneys and answered a series of questions about whether he understood the plea, saying "yes, sir" and "no, sir" to Bollinger.
Bollinger has barred attorneys on both sides from speaking to the media.