The Lehigh County district attorney's office says Jamie Silvonek entered the plea Thursday and will be sentenced to 35 years to life in prison.
Senior Deputy District Attorney Jeffrey Dimmig says as part of the deal, Silvonek will be eligible for parole after serving 35 years.
She was accused of conspiring with 21-year-old Army Spc. Caleb Barnes via text to kill her mother in the driveway of her home near Allentown.
Fifty-four-year-old Cheryl Silvonek was beaten, choked and stabbed in her SUV.
Prosecutors say she'd told her daughter to end the relationship because Barnes was too old.
Silvonek has been charged with first-degree murder, criminal conspiracy, abuse of a corpse and tampering.
Silvonek's boyfriend, Caleb Gregory Barnes, 21, was charged with homicide, abuse of a corpse, tampering with physical evidence and criminal conspiracy. Barnes's case is scheduled for trial on April 11, 2016.