Devon Jenkins, 16, was waiting in the car while his friend, Brok Junkermeier, stabbed and strangled 79-year-old Lila Warwick inside her home in Willmar this summer, according to court documents.
Tuesday morning, Jenkins pleaded guilty to unintentional second-degree murder, the West Central Tribune reports.
As part of a plea agreement, Jenkins will receive an extended juvenile jurisdiction sentence until he's 21 years old. If Jenkins violates the conditions of his juvenile sentence, he will face an adult sentence of 15 years in prison, the newspaper says.
Jenkins was one of three teens charged in Lila Warwick's death.
Prosecutors say Robert Warwick, 17, was the "mastermind" behind the plot to rob and kill his grandmother.
Robert Warwick and Junkermeier, 19, will be tried as adults and were indicted on first-degree murder charges in September.
Jenkins told a judge that he and Junkermeier smoked marijuana before going to Lila Warick's home during the early morning hours of July 29, the newspaper reports.
Armed with a long sword, Junkermeier said he was going to steal from Lila Warwick and kill her, according to Jenkins.
Junkermeier had even offered Jenkins $5,000 to come along, but Jenkins refused. He waited in the car where he fell asleep and woke up to Junkermeier returning to the vehicle, covered in blood.
Jenkins will remain in custody at Prairie Lakes Youth Detention Center in Willmar until his sentencing on Jan. 27, the newspaper says.
Junkermeier's trial has been scheduled for March 25.