Elmer Carroll was pronounced dead at 6:12 p.m. Wednesday following an injection at the state penitentiary at Starke, Gov. Rick Scott's office said.
The 56-year-old inmate was convicted of first-degree murder and sexual battery in the death of Christine McGowan. The girl lived with her family next door to a halfway house for homeless men where Carroll was staying in Apopka.
During Carroll's two decades on death row, his lawyers argued he was too mentally ill to stand trial or be subjected to the death penalty. The Supreme Court denied the petition.
Carroll previously had been imprisoned twice for indecent assault on a child.