The jury rejected defense arguments that Eddie Ray Routh was insane when he shot and killed Chris Kyle and his friend, Chad Littlefield.
Littlefield's mom addressed reporters following the ruling Tuesday night.
"We've waited two years for God to get justice for us on behalf of our son ... and we're so thrilled that we have the verdict that we have tonight," said Judy Littlefield.
Kyle and Littlefield had taken Routh to the shooting range at Rough Creek Lodge and Resort on Feb. 2, 2013, after Routh's mother asked Kyle to help her troubled son.
Defense attorneys noted that Kyle had described Routh as "straight-up nuts" in a text message to Littlefield as they drove to the luxury resort.
A resort employee discovered the bodies of Kyle and Littlefield about 5 p.m. later that day; each had been shot several times. About 45 minutes later, authorities say Routh pulled up to his sister's home in Kyle's truck and told her he had killed two people. Routh later took off and led authorities on a chase before the truck became disabled and he was arrested.
His trial has drawn intense interest, in part because of the blockbuster film starring Bradley Cooper based on former Navy SEAL Kyle's memoir about his four tours in Iraq.
During the trial, defense attorneys argued that at the time of the killings, Routh was suffering from Post Traumatic Stress Disorder. Prosecutors argued that, regardless of his condition, he still knew right from wrong.
Kyle's widow, Taya, became overwhelmed with emotion during court as she recounted the last time she saw her husband.
"We just said we loved each other and gave each other a kiss and a hug like we always did," she testified, choking back tears.
Prosecutors did not seek the death penalty against 27-year-old Routh, which means he'll spend life in prison without the possibility of parole.
His defense team said they would appeal the conviction.