Martin Johnson, 20, killed Erin Tomkins at her Sheffield home- causing numerous injuries to the head and face.
A post-mortem examination found that Erin had also suffered broken limbs and four fractures to her spine in the weeks leading up to her death.
Johnson, a former chef, told police he had been playing with Erin Tomkins at her home when she suddenly collapsed in front of him.
During his appearance at Sheffield Crown Court, Johnson was found guilty of murder after less than two hours of deliberations, along with two counts of causing grievous bodily harm with intent.
He will have to serve a minimum of 19 years behind bars.