Lilly Hanrahan died two days after being found collapsed with six broken ribs in Northfield, Birmingham.
Sean Sadler, who was in a relationship with her legal guardian at the time, claimed she had just fallen asleep and would not wake up.
Experts found 40 recent injuries on the little girl’s body after her death.
Sadler, 31, denied murder and wounding with intent, but a jury at Birmingham Crown Court believed the prosecution’s case her injuries were likely caused by being shaken and thrown against a surface, such as a settee’s armrest.
Lilly had been left in Sadler’s care by her guardian, who was appointed as her mother was unable to care for her.
A serious case review has been carried out into Lilly’s death and is expected to be published in June.
Before the fatal attack, the guardian had started noticing bruises on the youngster and even alerted her nursery to monitor her.
A post-mortem examination found three broken vertebrae in her spine and bleeding in her lungs from about three weeks before.
The dozens of recent and healing injuries were consistent with the youngster being gripped with excessive force or slapped, West Midlands Police said.
Sadler, of Coriander Close, Rubery, is due to be sentenced on Friday.
Det Con Jo Buchanan worked on the case as a family liaison officer and said it had been a “very difficult” one.
Prosecutor Hannah Sidaway said Lilly had been described as a “happy and bubbly little girl”.
She said the case itself had been “extremely complex”, involving a number of medical experts “to give evidence about Lilly’s injuries and to rule out non-accidental causes.