"I believed it was either him or me," Johnson said.
Johnson also was charged by the Chambersburg Police Department with robbery and theft in the death of Street, charges that were to be dismissed as part of the plea agreement, Assistant District Attorney Jeremiah Zook said.
The shooting took place on the afternoon of Feb. 1, but Johnson fled in the Excursion. Street's body was found in the vehicle the next morning in Harrisburg, Pa., police said.
Zook said no guns connected to the shooting were ever found by police.
Johnson was captured in March 2008 in Philadelphia and brought back to Chambersburg to face charges he shot Street as Street sat in the passenger seat of the Excursion, which was owned by his girlfriend, Shelby Flythe, police said. Flythe testified in July at Johnson's preliminary hearing that Johnson was sitting in the back seat when she stopped at her East Washington Street home to use the bathroom.
Flythe testified that while she was in her house, she heard gunshots. Another witness testified to hearing the shots and seeing Johnson next to the vehicle with Street slumped over in the passenger's seat with smoke coming from a head wound.
Flythe testified she and Street had picked up Johnson at a bar earlier in the day and the three made several stops before going to her home. After the shooting, Flythe testified she saw her vehicle being driven away.
In August, the District Attorney's Office filed a notice of aggravating circumstances in the case, a legal requirement to seek the death penalty in Pennsylvania.
No one from Street's or Johnson's families was present during the plea hearing. Zook said his office had contacted Street's parents, who told him they would be unable to attend the hearing, but that they agreed with the plea agreement.
Walsh scheduled Johnson's formal sentencing for Feb. 25. A sentence of life without parole is mandatory for a first-degree murder conviction.