Gleason-Mitchell previously admitted causing or allowing the death of a child, cruelty to a child in relation to cocaine, and cruelty to a child in relation to cannabis.
Jeff was found guilty of these charges and a sentencing hearing is due on 13 December.
Isabella's body was found under a pile of blankets with "traumatic injuries" by a police officer.
The trial, which began on 8 October, heard from expert witnesses that she had suffered from "crash-like" injuries including broken and shattered bones.
Gleason-Mitchell and Jeff were arrested a day after the discovery of the Isabella's body, and the court heard the pair were picked up having had a night out at the Corn Exchange pub in Bury St Edmunds.
Jeff, who denied all the charges, had only been in Isabella's life for just 36 days before she died.
Members of Isabella's family sobbed in the public gallery as they learned of what happened to her in that time.