The mom of a 13-year-old Brooklyn boy violently robbed by a deranged vagrant before the nut allegedly slashed a woman Monday says she was shocked to learn he was still on the streets.

Fanny Cardenas told The Post on Tuesday that prosecutors assured her that violent ex-con Muslim Brunson would be institutionalized after attacking her son.

“He’s still on the street?” Cardenas said. “After he attacked my son, they told me he would be admitted to a mental institution.

“The Brooklyn DA’s Office told me that they would send him to a mental institution because he already had a history of mental illness,” the 38-year-old doctor’s office worker said. “They need to lock him up. It’s scary that he’s still on the street and people are exposed to a man like that.”

Career criminal Brunson, 46, is facing attempted murder and assault charges in the Monday afternoon attack in Soho on Arizona native Megan Berg, 25, who was randomly slashed in the neck with a broken bottle. She is in the intensive care unit at Bellevue Hospital.

Brunson was supposed to get locked up after the Sept. 17, 2019, attack on Cardenas’ young teen son on a Queens-bound C train between Liberty and Van Siclen avenues in Brooklyn.

Police said Brunson snatched the boy’s phone and scuffled with the youngster when he tried to get it back.

The maniac caught a break when he was sentenced to a mental health program as an alternative to jail, leaving him free to roam around.

Timeline of a ticking time bomb

• Aug. 17, 2019: A 911 call is made involving Muslim Brunson, with him hearing voices and wanting to harm others, according to law enforcement sources. He is taken to Beth Israel Hospital in Manhattan.

• Sept. 17, 2019: Brunson accosts a 13-year-old straphanger on a Brooklyn subway, robbing the kid of his iPhone and shoving him off the train when he tries to fight back. Brunson is freed under a special no-jail program that instead mandates mental health treatment — then goes before a judge 36 times over the next three years to ensure he is complying.

• Dec. 9, 2020: Brunson is reportedly hearing voices telling him to harm others and is transported to Beth Israel.

• Dec. 16, 2020: He is hearing voices telling him to hurt other people and to not take his medication and is taken to Lincoln Hospital in the Bronx, sources said.

• June 18, 2022: Another EDP report involving Brunson is made, saying he needs medication. He is taken to Beth Israel.

• July 4, 2022: Brunson violently shoves a female civilian NYPD employee on the subway, breaking her eye socket, sources said.

• July 6, 2022: He is tracked down by cops in the subway system for the crime and proceeds to kick one of the arresting officers near the right knee and spit on police, netting another felony assault rap. Brunson is booted from the mental health program, his 2019 and 2022 cases are rolled into one and he is convicted and starts serving a year behind bars.

• Aug. 18, 2023: Brunson is released from prison.

• Jan. 24, 2024: He is accused of public lewdness.

• Feb. 18, 2024: Brunson’s parole ends.

• April 7: He allegedly throws a glass bottle at a woman on the street in Soho, then picks up shards and slashes another female passer-by in the neck, critically wounding her.

That came to an end in 2022 when he randomly slammed an off-duty NYPD civilian employee into a pole on the subway in Manhattan.

That case landed Brunson at the Sing Sing Correctional Facility until his parole in February 2024.

On Monday, he allegedly unleashed the new attack on Berg.

Cardenas said it was a chilling update.

“He shouldn’t be on the street because he could attack anybody,” she said Tuesday. “Something’s not right with his brain. They need to help him in some way.”

A ​rep for the Brooklyn DA’s Office said Tuesday that the distraught mom wa​s told at the time that “the case would be handled in Mental Health Court with a program,” and that “she was amenable” to the decision.

Brunson was sent to state prison when he was re​arrested​ in 2022, the ​representative noted. 

Additional reporting by Kyle Schnitzer