The victim was 73-year-old Lucia Maria Guajardo. The Hereford Brand reports Guajardo was stabbed last July 30, but lived long enough to call 911 to report "someone tried to kill me."
Prosecutors presented evidence of the teen's drug use. DNA evidence also linked Grajeda, who was convicted late Thursday, to the attack.
Investigators say Grajeda was suspected of forging checks stolen from her grandmother in the weeks before the killing.
A teenage co-defendant faces trial later.
Hereford is 45 miles southwest of Amarillo.
