The young mom fatally shot while pushing her 3-month-old daughter in a stroller on the Upper East Side was allegedly abused by the baby’s father — who is now being sought for questioning by police, law-enforcement sources and family members told The Post on Thursday.

Azsia Johnson, 20, had been allegedly assaulted by her ex-boyfriend at least once and was wracked with fear when he found out she’d given birth and had been released from the hospital, according to her heartbroken mom, Lisa DeSort.

“He was stalking her to the point he knew when she was discharged from Mount Sinai hospital with her daughter,” DeSort said.

The young mom even moved to a domestic abuse shelter at one point to “keep safe,” but “somehow he found out where my daughter lived,” DeSort added.

Despite her fears, Johnson texted relatives she planned to meet the infant’s dad on Wednesday night because she “felt bad” that he wasn’t in their daughter’s life and wanted her to know him, sources and relatives said.

She was walking with her baby girl in a stroller at around 8:25 p.m. that night near the corner of Lexington Avenue and East 95th Street when an assailant dressed in a black hoodie and sweatpants shot her once in the head and fled.

“She was targeted,” a high-ranking police source said. “It was a close head shot and there were powder burns.”

Law-enforcement sources confirmed that Johnson’s shooting death was “domestic” in nature. No arrests have been made and the gunman’s identity has not been released.

Investigators are looking to speak to the ex in connection with the murder, and he is considered a person of interest in the slaying, according to the sources.

Sources said the ex was actively calling family members while on the run to check on the baby, who was unharmed in the shooting.

DeSort said her daughter was “wonderful” and “an exceptional young lady” who had big dreams and was just getting enrolled into nursing school.

“She worked hard and had dreams of becoming a pediatric nurse in the near future,” DeSort said, adding her daughter was currently on maternity leave from her job as a home health aide taking care of the elderly.

“She was a good girl who was never in trouble. She was respectable.”

The tragic young woman reported to the cops on New Year’s Day that her ex had assaulted her at his home in Jamaica, Queens, while she was six months pregnant with his child, according to DeSort and police sources.

Following the altercation, DeSort alleged, the ex-boyfriend continued “stalking” and harassing Johnson. She and her mother called police for help, but he was not arrested, DeSort claimed.

“Let it be known that my daughter had a domestic violence case out on the father of her child,” DeSort, a retired EMT of 23 years who worked during the Sept. 11 terror attacks, told The Post.

“We called the precinct numerous times to tell the [domestic violence] unit that he was stalking and [harassing] her. Even [though] they knew what apartment he lived in, they failed to apprehend and arrest him,” she continued.

“The city failed to protect my daughter.”

Law-enforcement sources said police put a warrant out for the ex-boyfriend’s arrest after Johnson reported that he had allegedly assaulted her on Jan. 1.

Cops tried to find him but couldn’t, the sources claimed, while noting that the crime he was charged with, misdemeanor assault, is not bail eligible and he would’ve likely been released from custody soon after the arrest.

Numerous studies have shown that domestic abusers are less likely to re-offend after they enter the criminal justice system, the Department of Justice has said.

“The police failed my daughter,” Johnson’s grief-stricken mom said.

The NYPD didn’t return a request for comment.

A law-enforcement source said cops tried to follow up with Johnson after the domestic violence incident but couldn’t locate her, which is “not unusual in the spectrum of DV cases.”

Meanwhile, Mayor Eric Adams placed the blame on gun violence at a candlelit vigil for the young mom Thursday night. 

“The [police] commissioner and I have been in too many hospitals talking to family members saying sorry for what’s happening,” he said as he stood next to the grief-stricken mother. “We really need all of our city to unite and send a strong message that we don’t want to wake up to gunshots. We don’t want to see blood on our streets. We don’t want to lose innocent families and children.”

He insinuated that the NYPD was doing its job to keep guns off the streets, while the courts’ efforts are lacking.

“Every arm of our criminal justice system must be part of this battle and right now I don’t feel like they are,” Adams said. “There’s a different reality of what’s playing out on the streets and what is being done in the other pieces of our criminal justice system.”

The mayor added that the tragedy felt personal. He said he worked with DeSort when she was an EMT and he was a cop. “It hits so close to home,” Adams said. DeSort, who spoke at the vigil after Adams, doubled-down on her demands for the city and police to take domestic violence seriously. 

“[I]t’s not guns, it’s the people shooting the guns,” she said, adding that Adams is doing the best he can. “It’s about taking domestic violence serious[ly],” DeSort shouted. DeSort told the Post earlier in the day that her daughter’s ex had been keeping tabs on his estranged girlfriend.

When Johnson left the hospital after giving birth, “she called me and she was upset because she said, ‘Mommy, how did the hospital tell him I was discharged and to where?’

“He told her he knew where she was. He had the exact date and time she was discharged,” DeSort said by phone.

Johnson’s infant is now being evaluated by the city Administration for Children’s Services, DeSort said.

“I was going to take care of her kids while she went to school,” the grieving woman said of her slain daughter, who also had a toddler from a previous relationship. “We called her ‘bougie’ because she liked the nicer things in life and she worked hard to get them.”

DeSort said she decided to donate her daughter’s organs Thursday afternoon.

“I harvested my daughter’s body to save someone else. She would want that,” DeSort said.

Johnson lived with her mother and sisters in an apartment in the Bronx but had been in and out of the shelter system over the last year, according to law enforcement sources, DeSort and a neighbor at the building.

“Last night, someone came and told her mother … and she just started screaming, ‘My baby’s dead, my baby’s dead!’ … then they all left,” neighbor Ebony Paul, 40, told The Post.

“Azsia just had that baby three months ago,” she said. “She’s a beautiful little girl.”

Paul said DeSort’s pained screams filled the building when she was told of her daughter’s death Wednesday night.

“I could hear her screaming last night and then you didn’t hear anything because everyone must have left to go check out what’s going on with her,” she said. “I’ve been up ever since.”

Another neighbor of the Wakefield home remembered Johnson as “a perfect mom” who was “always smiling.”

“She took care of the kids,” said the man, who asked to be identified only as Ron. “The baby would always touch my fence and want to play with my puppy. It’s very sad.”

“So much guns out there,” he added. “I’m scared.”

Relatives have set up a GoFundMe page in Johnson’s honor.

At the end of the vigil Thursday night, Johnson’s distraught family released blue, white and red heart-shaped balloons into the sky. Through tears, they shouted “Aszia, we love you!”

Earlier that morning, police were seen towing a Lexus SUV with an apparent bullet hole above the front right wheel arch that had been parked overnight near the scene of the shooting.

The vehicle’s owner, Julio Cruz, 62, hopes it’ll help cops track down the killer.

“The officer said the hole in my car is the size of a bullet and the bullet is probably still inside the car,” Cruz told The Post, adding he didn’t see the hole on Wednesday afternoon.

“They will inspect the car and try to find the bullet … I hope they find something that will help this case.” 

Additional reporting by Kevin Sheehan, Reuven Fenton, Jorge Fitz-Gibbon and Steven Vago