Ferrell Dardar took a plea deal. He was sentenced to 60 years in prison on Monday.
Dardar and Philip Battles were both charged with capital murder in her death. A third man, Marco Miller, was also charged.
Dardar will be eligible for parole in 30 years. There's no word whether Battles or Miller will also accept a plea deal.
The deadly robbery happened on Nov. 14, 2016 in north Harris County.
Authorities say Ava's mother, Diana Gomez, Ava, and Gomez's two other children had just returned home from the grocery store when three men walked up to them and tried to steal Gomez's purse.
"I saw my daughter on the floor and she wasn't able to breathe no more. She was trying. I was screaming 'Ava!'" Gomez told Eyewitness News two weeks after the shooting.
Gomez said the suspects started shooting and didn't even give her time to give them her purse. She was shot seven times.
Her 10-year-old daughter Betzy was grazed by a bullet. Ava was killed.
On Monday, a dozen members of Ava's family, including her mother, were in court. They did not want to say anything until the plea agreement was made official.
Gomez read an impact statement in court.
"It breaks my heart. I will never see her again, never hear her voice again, never hear her call me mommy," the statement said. "Her passing has left a hole in our hearts that can never be filled."
Ava's family also showed three pictures of her while the statement was read. Two of those photos showed a smiling girl. The third photo below the others was a picture of Ava in her casket.
Gomez told the court, "The way I remember her everyday is the photo on the bottom. That is what you have done to me, to her, to her family."
Battles and Dardar were also charged with capital murder in the death of Melvin Bonilla-Fuentes, which happened about a week before Ava's murder.
According to police, Bonilla-Fuentes was confronted during a robbery near his apartment on Goodson.
Police said there was some sort of struggle and he ended up being shot.
Battles was charged with capital murder a third time in the death of 62-year-old Ignacio Ortega, who was killed while gardening.
Investigators at the time said Battles confessed to the crime.
MORE: 'What I want is the death penalty': Mom of murdered 4-year-old speaks about case
Related stories on Ava Castillo case:
HCSO: Robbers fatally shoot 4-year-old over mom's purse
Mother of 4-year-old shot to death during robbery tells her story
EXCLUSIVE: Philip Battles talks to ABC13 in jailhouse interview
Follow Jeff Ehling on Facebook and Twitter.