Prosecutors said the crimes occurred while Stepp babysat Yarley inside his south Raleigh apartment in November 2009.
Stepp was also found guilty of a first-degree sex offense with a child.
The trial now moves into the penalty phase - basically a mini-trial where it will be determined if Stepp will face the death penalty.
Lawyers for Stepp, an Iraq war veteran, said he was suffering from post-traumatic stress disorder and was self-medicating using alcohol and drugs the night the child died. They said he killed the baby, but didn't intend to and didn't rape her.
The jury had to listen to extremely graphic testimony from experts who described injuries to the child's genitals doctors found when she arrived at the emergency room. The medical examiner said it was a blow to the head that killed her.