The baby-faced teen charged with fatally stabbing his 16-year-old Harlem girlfriend was already on probation for robbery and being sought for three other heists when he allegedly murdered his victim, police sources told The Post on Tuesday.

The disturbing revelation comes as new grisly details also emerged in Sunday’s slaying of Saniyah Lawrence.

Lawrence was stabbed once in the neck after getting into an argument with her beau, Zyaire Crumbley, 18, over another guy, according to sources.

Crumbley was allegedly upset that Lawrence received a phone call from another boy, and he grabbed her phone from her while the pair was in their apartment, sources said.

Crumbley, whose nickname is “Ziggy,” allegedly threatened Lawrence when she tried to get the phone back, telling her, “Stay away from me before I stab you!”

After Lawrence was stabbed, Crumbley left the apartment, and the mortally wounded Lawrence screamed and called 911 while Crumbley then tried to kick his way back in, sources said.

Lawrence was eventually rushed to Harlem Hospital, where she was pronounced dead.

An autopsy on her body found she died from a single stab wound to the right side of her neck just above her collar bone and suffered internal bleeding as a result of the wound.

The medical examiner also determined that the teen was not pregnant — despite her claims that she was when she filed a sexual misconduct complaint against the older Crumbley in October, noting that she was 15 and underage at the time, sources said.

Cops searching the Seventh Avenue apartment after Sunday’s murder found among other things a paper bag covered with blood and various knives and scissors, according to the sources.

Crumbley was still on probation stemming from a 2020 robbery conviction at the time of the fatal stabbing, the sources told The Post.

He also has five other prior robbery arrests dating to 2016 and was being sought for three other robberies in Manhattan, also all from August 2020, sources said.

In addition, the troubled teen was facing the Oct. 27 complaint over allegedly having sex with Lawrence when she was just 15, sources said. The complaint was filed one day after Crumbley’s mom allegedly threw her son out of the family home. He had also run away from home in 2015, sources said.

In December 2015, he also was allegedly hospitalized following a call for an emotionally disturbed person at a city high school for threats against classmates, sources said.

Crumbley was ordered held without bail at his Tuesday night arraignment.