She called 911 the next morning claiming that someone stole her vehicle with the child inside.
"Sadly, I didn't think she was alive when I got to her", Dunn said in a video produced by the Seminole County Sheriff's Office.
The Seminole County Sheriff's Office posted a photograph of the emotional reunion on its Facebook page.
The body camera footage shows Dunn running while holding the girl in his arms.More news: Focus shifts to rescues as rain abates in India's flood-hit Kerala
The child became responsive once Dunn blasted the air conditioning with her in his patrol auto.
"I felt for a pulse", he continued. So I took her out of the auto seat and she was just limp. He can be heard in the video saying: "It's OK, baby".
Her eyes started fluttering, he said, and he could feel her heart beating fast.
The deputy carried her to the trauma center and left after realizing there was not much else he could do.More news: ICE detains man driving pregnant wife to hospital to deliver baby
"I remember one thing distinctively", he said. "She asked if that was mine, and I said, 'no, it's yours, '" he said.
The girl was released from the hospital three days after the incident and Dunn was able to meet her.
"Sometimes we're able to have a moment like this that makes it all worthwhile", he said.
The child, despite being in the auto for roughly 12 hours, survived.More news: Nobel Prize-winning author VS Naipaul passes away at 85
NBC 2 notes that law enforcement officials said the mother, 33-year-old Casey Dyan Keller, "took three children to a liquor store late Saturday and returned back to her apartment around 11:15 p.m. Keller allegedly brought two of the children inside, but not the three-year-old".