The Maricopa County Superior Court jury ordered the life sentence without the possibility of parole, WPNX-TV, Phoenix, reported.
Dieteman, 31, pleaded guilty earlier to two murders and conspiracy to commit first-degree murder. Other charges against him were dropped in exchange for his testimony against co-defendants Dale and Jeff Hausner, the TV station said.
The trio were charged in a string of shootings that began in May 2005 in which eight people were killed and 17 were wounded. Ten animals also were shot.
Dale Hausner was given six death sentences, two life sentences and more than 900 years in prison after being convicted of 80 charges.
Jeff Hausner was not charged with murder, but was given 25 years in prison for two non-fatal stabbings.