Jaclyn Renee Pennington, 26, entered her plea in 271st District Court in Decatur in exchange for the 25-year sentence. The case had been set for trial next month.
The case involves the death of Pennington's daughter, Taylor-Rae Baker. The exact circumstances of Baker's death are not known.
"We've got 10 different stories from (Pennington)," said District Attorney Greg Lowery.
The only witness at the home was Pennington's other daughter, who was 5 at the time.
Deputies and medics responded to a medical call at Pennington's home on Farm Road 51 south of Decatur Feb. 17, 2008, and found Baker lying on an air mattress.
Officers noted that the girl had several large bruises and cuts on her body.
Baker was taken to Wise Regional Health System, then flown to Cook Children's Medical Center in Fort Worth where doctors performed several surgeries to try to save the child's life.
The Tarrant County Medical Examiner's Office ruled the death a homicide, saying blunt-force trauma severed an artery, and the child bled to death.
Investigators talked to family members, and Pennington later confessed to throwing her daughter into a fireplace, according to the arrest affidavit.
Pennington was arrested two weeks after Baker's death, and she has remained in jail.
In May 2008, a Wise County grand jury indicted Pennington for first-degree injury to a child.
Lowery said that the case was re-indicted last week as a felony murder, and a plea agreement was reached.
Pennington will have to serve at least 10 1/2 years of her sentence before she is eligible for parole, he said.
Baker's family is satisfied with the judgment, Lowery said.
"I've spent many hours with the Bakers on this case. They are angry. They want their child back. And they want (Pennington) to feel the pain they are feeling," he said.
