A Nassau prosecutor told jurors Monday that a toddler suffered and died slowly at the hands of a Uniondale man who unleashed a “depraved” attack on his girlfriend’s 18-month-old son while baby-sitting the child in his home.

Lord Pardo, 28, is on trial and accused of murder and manslaughter in the Oct. 11, 2015, death of Mason Robinson of New Cassel.

But Pardo’s attorney, Jeffrey Groder, said during opening statements in a Mineola courtroom that while the child’s death was a tragedy, evidence in the case would lead jurors to a not-guilty verdict.

Police called Pardo “a monster” at the time of his arrest, with prosecutors saying that Mason was “severely beaten” and that he died after suffering a lacerated liver and lung, a fractured skull and broken ribs.

After the beating, Pardo “sat there and let Mason die,” Assistant District Attorney Jessica Cepriano said Monday.

“Mason’s injuries, they speak for him. They call for one specific, tragic and bold conclusion: that the defendant killed him,” she added.

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Cepriano said the toddler already was dead by the time Pardo left his home with the child to drive him to the mother’s Jericho workplace, after never calling 911 to seek help. She said Pardo also disposed of a bag of tissues with Mason’s blood on them before leaving.

Groder told jurors Mason fell off a bed when Pardo turned away to get powder while changing the child’s diaper. The toddler then fell from Pardo’s arms after the man picked up the child to try to comfort him, Groder said.

After that, Pardo — a Haitian immigrant and factory worker — fed the child and put him down for a nap, according to Groder. But he said Pardo couldn’t wake Mason and drove the child to his mother not knowing Mason was dead.

Groder also said the prosecution had to hire an expert medical witness “to fill in the gaps” in what the medical examiner would testify to in order to advance the government’s theory of what happened that day.

Police previously said Pardo, who came to the United States from Haiti in 2009 and has no criminal record, also was minding his own 3-week-old daughter while baby-sitting Mason.

Authorities said Mason’s mother left him in Pardo’s care about 7:30 a.m. before Pardo texted at 1:40 p.m. to say Mason wasn’t waking up from his nap. Mason’s mother, Marie Eresthine, then 23, asked Pardo to bring her son to the grocery store where she was working after she realized the child was unresponsive, authorities said.

Law enforcement officials said the child — his body already stiffening from rigor mortis — got CPR at the store, before his death pronouncement at Syosset Hospital at 2:57 p.m.