Bonser, of Cusworth House, Doncaster, denied murder. She has a long history of mental health problems and cannabis abuse and her defence team claimed she is a paranoid schizophrenic.
Her barrister, David Fish, asked the jury of seven women and three men to find his client guilty of manslaughter due to diminished responsibility or lack of intent.
But prosecutors argued the 26-year-old suffers from a personality disorder, not a psychosis, and was guilty of murder. The jury agreed.
Sentencing her, Judge Mr Justice Cranston said: "Why did Hannah Bonser do this to Casey - a young girl, a complete and wholly innocent stranger?" He said she suffered a "horrific death".
Speaking outside court, Mr Kearney said: "We think the jury has come to the correct decision in convicting Bonser with the murder of Casey. There was never a doubt in our minds this was a calculated and deliberate act and therefore deserves the maximum sentence our judicial system allows. However, no sentence will bring Casey back to us and the severity of Bonser's actions mean we will never have the privilege of her in our lives again."