Jefferson, of Birch Court, Walsall, was also found guilty of two counts of causing grievous bodily harm with intent.
Clark, from Lichfield, in Staffordshire, was also found guilty of two counts of causing or allowing serious harm to a child, but cleared of two further charges of causing grievous bodily harm with intent.
Charges of murder or manslaughter against her were dropped by prosecutors last week, during the seven-week trial.
Jefferson, who wore a grey tracksuit and blue Adidas jacket, shook his head in the dock as the jury of eight women and four men returned their unanimous verdicts.
Clark, who wore a green jumper, black trousers and a beige coat in the dock, put her hand to her face.
The court previously heard Darcy-Leigh had suffered more than 40 rib fractures, as well as broken legs.
The jury was told Clark was struggling in the run up to her death and that she had engaged in a number of heated exchanges with Jefferson.
These included text messages about how she was exhausted and felt he was not supporting her.
Following the verdicts, the Staffordshire councillor responsible for child services, Nick Lakin, said a council review of the case would be “published in due course”.
He said changes had been made “quickly” following Darcy-Leigh’s death, but accepted “more could, and should, have been done by those who had the opportunity to intervene while Darcy Leigh was alive”.
He apologised for “the failure to act decisively in 2022”.
The judge, Mrs Justice Brunner KC, said the pair would be sentenced on a date to be fixed.
The jury foreperson, who gave the verdicts in court, wiped her eyes as the judge thanked them for their attention to the “upsetting” case.
