The unanimous vote on Michael King's punishment came after three hours of deliberations, CNN reported. Under Florida law, the trial judge makes the final decision.
Denise Lee, 21, the daughter of a police detective, was abducted from her home in January 2008. Investigators said King drove her to his house, where he raped and shot her, and dumped her body in a ditch.
During the penalty phase, King's relatives testified the 38-year-old man suffered a severe head injury at the age of 6. His brother, Greg, said King would occasionally become violent and said he suffered from headaches and a buzzing noise in his head.
The jury was also told King's IQ is 76. Those with an IQ of 70 or less are considered retarded.
Two jurors who spoke to the news media said neither the head injury nor King's IQ was given much weight.
During the trial, jurors heard a call Lee made to 911 after the kidnapping.
"Oh, please, I just want to see my family again! Let me go!" she begged.