Michael Moore had pleaded not guilty to charges of sexual assault and murder.
Prosecutors didn't seek the death penalty at the family's request, which is why Moore's conviction means an automatic life prison term.
Prosecutors said that DNA and other evidence tied him to the death of Alicia Moore, whose body was found in a trunk on the side of a road in Van Zandt County more than two years ago.
According to the arrest warrant affidavit, Alicia told her best friend she was going somewhere with her uncle before she disappeared.
The court documents say a DNA sample from Moore matches the rape kit samples taken from Alicia's body. And, traces of animal hair found in the truck where her body was discovered may be linked to a dog found in Moore's backyard.
Prosecutors argued that Moore was guilty because he didn't use his phone for an extended period around the time Alicia was killed on November 4, 2012. He began using it again the next day, around the time her body was left in the trunk.
But defense attorneys said Moore is gay. They claimed evidence in the case was planted.
They had Alicia's aunt confirm that her family called police for help because the teen had being engaging in repeated sex with multiple older men.
Alicia's family was shocked to learn that Moore was accused.