Chastinea T. Reeves, 17, changed her plea Tuesday under an agreement with prosecutors in exchange for a 45-year prison sentence.
Judge Diane Boswell took the plea agreement under advisement. Reeves is scheduled to be sentenced June 12.
She had been scheduled to go to trial June 3 in the February 2017 slaying of 34-year-old Jamie Garnett at their Gary home.
Reeves, who was 15 at the time of the slaying, was charged as an adult in Lake Superior Court.
Defense attorney John Cantrell said he didn't think Reeves had any chance of being acquitted at trial because prosecutors had the knife used to kill Garnett with Reeves' fingerprint in blood.
An Amber Alert was issued for Reeves after her mother was found dead and the teen could not be found.
Reeves and her younger sister went to their aunt's home in a Gary housing complex in the 2000-block of Maine Street after her mother was killed.
"Two children came to my door a little after 2 a.m. and they said someone had come in, somebody had did something to their mom," said aunt Kelli McMillan. "She run out the back door when the police come."
"The next morning, when I was getting my children off to school, that's when everyone was saying they found her dead in her home," said Sharmalisa Anderson, who lives nearby. "I knew her, and that's really sad. She also has a 4 year old. I've known her for a long time. She didn't deserve that. No matter who did it, she didn't deserve that."
The Associated Press contributed to this report.