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.