Kaylee-Jayde Priest was found dead at the flat in Solihull where she lived with her mother, Nicola Priest, 23, on 9 August last year.
Priest and lover Callum Redfern, 22, were cleared of murder but convicted of manslaughter.
The youngster died from serious chest and abdominal injuries.
The child’s mother rang 999 but a jury convicted Priest after hearing the youngster had been “dead before the call was made”.
Medical examinations later showed she had also suffered historical injuries including broken ribs, lower leg fractures and a broken sternum, Birmingham Crown Court heard.
Priest and Redfern were at the time in a “close sexual relationship”.
Jurors had heard how the mother would hit Kaylee around the head, while the youngster was also heard crying “in a fearful tone”.
On one occasion, neighbours in a flat below at Kingshurst House, Solihull, recalled hearing a bang above and then the girl crying, before allegedly hearing Priest say: “I’ll just say she fell off the bed.”
The same residents claimed they noticed that “when Kaylee cried, the response appeared to be to drown out her crying with music”.
In a text message exchange on 24 July last year, days before the youngster’s death, Priest told Redfern: “I’m gonna kill her… because she keeps leaving the living room or going in the kitchen, so I’ve paled [hit] her one and smacked her for [dirtying] her nappy.”
Redfern said: “Good – give her one from me.”
Priest replied: “I will, babe.”
The mother was also found guilty of cruelty to a child, relating to the youngster’s historical injuries, but Redfern was cleared of that charge.
Priest, of Poplar Avenue, Edgbaston, Birmingham, and Redfern, of Temple Street, Dudley, will be sentenced on Friday.