Kelly, 37, pleaded guilty last year to taking Cleo from the Blowholes campsite, about 960km north of Perth, on October 16, 2021.
Cleo was missing for 18 days before finally being found by police alone in a room at a property in nearby Carnarvon on November 3.
The dramatic rescue and Cleo's confirmation of her name was captured by an officer's body-worn camera and subsequently made news headlines around the world.
In the District Court of Western Australia, Chief Judge Julie Wager sentenced Kelly to 13 years and six months imprisonment, describing the fear, distress and trauma caused to Cleo and her parents as "immeasurable".
"Eighteen days without contact or explanation, and with hours totally on her own and no access to the outside world, would have been very traumatic," the judge said on Wednesday.
"In the world of a four-year-old, one day is a very long time. In the world of a four-year-old, 18 days is a very, very long time indeed."
Kelly, who sat silently in the dock wearing a green shirt, will be eligible for parole after serving 11 years and six months.
Cleo's mother Ellie Smith and stepfather Jake Gliddon were in the public gallery for the long-awaited sentencing hearing.
The court heard Kelly had a "significant interest" in dolls and Facebook pages with "fantasy children".
He arrived at the campsite looking for items to steal before coming across the Smith family's two-room tent, where Cleo and her younger sister were sleeping in a separate compartment.
Kelly made the opportunistic decision to snatch Cleo, lifting her up along with her sleeping bag and carrying her to his car in "relative silence" and going unnoticed by her sleeping parents.
He kept Cleo at his Carnarvon home for the entirety of her captivity, locking her in a bedroom for much of that period after modifying the door and leaving her home alone for long periods of time while he went shopping and visited relatives.
Kelly told police he had felt bad for detaining Cleo but also admitted becoming angry with her, saying he had "roughed her up a few times".
"I wasn't planning to keep her forever, you know. I was getting guilty every day and it was just more weight on my shoulders," Kelly said during a police interview, according to Judge Wager.
The judge said Cleo had pleaded to be returned to her parents and Kelly, who was aware of the desperate search for Cleo, had used a loud radio to drown out her pleas.
"When the young victim heard her name on the radio, she said 'they're saying my name'," Judge Wager said.
Kelly later told police he had been injecting methamphetamine at the time, including shortly before he arrived at the Blowholes campsite.
Cleo's parents had woken at the campsite, "not knowing if she was alive or dead for the next 18 days".
"They didn't know what happened to her or if she would ever return," Judge Wager said.
"(They) were sad, scared and confused. They described being too fearful to sleep ... of feeling completely empty and broken.
"They stayed at the place that caused them so much pain, hoping their little girl would be located."
Judge Wager noted Kelly's deprived childhood and complex personality and developmental dysfunction.
She accepted Kelly would have been "far less likely" to commit the crime if he had not been disinhibited by illicit drug use.
