Hannah Bonser has been jailed for life after being found guilty of the murder of 13-year-old Casey Kearney. She has been told she will serve a minimum of 22 years in prison.
Casey was stabbed in a random attack in Elmfield Park, Doncaster, South Yorkshire, on Valentine's Day.
The judge, Mr Justice Cranston, told Sheffield Crown Court Casey's family had reacted with "enormous dignity" as they listened to the details of her death.
He added: "Casey did not deserve this. She had done nothing wrong."
Casey Kearney's family has said: "This was a calculated deliberate act...No sentence will bring Casey back.
"The severity of Bonser's actions means we will never have the privilege of her (Casey) in our lives again."
The judge, Mr Justice Cranston, has said he will pass sentence on Hannah Bonser, for the murder of Casey Kearney, at 2pm.
Bonser showed no emotion when the jury foreman returned the guilty verdict after two hours of deliberation.
She sat in the dock looking ahead, as she has done throughout the week-long trial, surrounded by four prison officers.
There were cries of "Yes" from the public gallery which was packed with Casey's family. Her father, Anthony Kerney, appeared to wipe away tears.
Many in the gallery were wearing orange ties and scarves - one of Casey's favourite colours - in memory of the teenager.
A woman with a history of mental health problems has been found guilty of murdering a 13-year-old stranger in a park.
Hannah Bonser, who is 26, stabbed Casey Kearney in a random attack in Elmfield Park, Doncaster, South Yorkshire, on Valentine's Day.
A jury at Sheffield Crown Court rejected Bonser's claim that she was guilty only of manslaughter due to diminished responsibility.
Bonser is due to be sentenced shortly.
A jury sitting at Sheffield Crown Court has found Hannah Bonser guilty of the murder of 13-year-old Casey Kearney.
Casey Kearney was found fatally injured in Elmfield Park, Doncaster, South Yorkshire, in February.