The heartless brute who randomly knocked out an 81-year-old woman on the Upper West Side was out on bail for an earlier crime: exposing himself to a mom and her two kids, Manhattan prosecutors said Friday.

Hansel Esparragoza, 37, had two open cases against him in Staten Island and was out on supervised release when he slugged elderly Geula Freeman at West 66th Street and Amsterdam Avenue in Manhattan on Sept. 13 in a shocking caught-on-video attack, authorities said.

The suspect already had multiple arrest warrants out for him for failing to show up to court in those cases when he decked the old lady, prosecutors said.

He was finally held on $75,000 bail in Manhattan Criminal Court at his arraignment Friday on assault charges for allegedly punching his defenseless victim.

Esparragoza — a Staten Island resident with 21 prior arrests in all — was busted July 27 when he exposed himself to a woman and her two children as he muttered, “Hey Mommy” outside a strip mall on Staten Island, Manhattan Assistant District Attorney Hunter Carrell said at Friday’s hearing.

In that case, Esparragoza was charged with child endangerment and public lewdness and freed on supervised release because the raps weren’t bail-eligible, officials said.

He was then re-arrested two weeks later for allegedly swiping a book from a Barnes & Noble store at the Staten Island Mall on Aug. 9.

The suspect was issued a desk-appearance ticket for petit larceny since those charges weren’t bail eligible, either.

“Much to the dismay of our office and the people of Staten Island, because of New York State’s disastrous so-called bail reform, none of the charges brought against this defendant are bail-eligible,” a spokesman for the Staten Island District Attorney’s Office said.

In Manhattan court Friday, prosecutors walked through the moment Esparragoza allegedly attacked the defenseless Freeman, who was walking her son’s Goldendoodle at the time.

Footage of the senseless assault shows Esparragoza first walk by the victim as she’s strolling with the dog, only for him to turn back.

“After taking a few steps, [Esparragoza], seemingly for no reason, turned back and began quickly approaching the complaint … before rearing back and punching her square in the jaw,” Carrell said.

Esparragoza’s blow knocked Freeman out cold. She suffered a fractured nose and bruising to her face and neck, according to prosecutors.

Freeman has told The Post that she has amnesia from the attack and can’t remember anything about it.

“It was such a surprise because it was described to me how it happened that I would not have been able to do anything,” she said.

Esparragoza’s lawyer, Mariah Martinez, said his client struggles from mental-health and substance-abuse issues, which have been going “unaddressed” during his recent arrests.

Prosecutors said Esparragoza has one prior felony conviction for attempted assault after he beat up a stranger who wouldn’t give up his bike.

Esparragoza was arrested Thursday in the Upper West Side attack and faces three years in prison if convicted of the top charge — assault in the second degree.

Manhattan Judge Melissa Lewis acknowledged at Friday’s hearing that Esparragoza needs mental-health treatment but called the allegations “egregious” before setting bail.

He’s due back in court Oct. 9.