The verdict came after less than a day of deliberations.
The same Snohomish County Superior Court jury convicted Opel of aggravated first-degree murder - the state's only potential death-penalty crime - on April 8, after deliberating about six hours.
Prosecutors alleged that Opel, 39 and a mother of three, orchestrated the April 2001 slaying of Jerry D. Heimann, a retired Boeing worker with terminal cancer who had hired her to care for his aging mother.
Heimann, 64, was beaten and stabbed by a group of teens - including Opel's then-13-year-old daughter - then buried in a shallow grave on the Tulalip Indian Reservation. His body was found on April 21, 2001, eight days after the killing.
Prosecutors told jurors Opel had promised the teens money and gifts for carrying out the killing so she could steal $40,000 Heimann had made from the sale of a house.
Prosecutors had sought the death penalty for Opel, who would have been the only woman on Washington's death row had the jury reached a unanimous verdict that she be executed. Aggravated first-degree murder, under Washington law, is punishable either by death or life imprisonment without parole.
Opel broke down in tears and hugged her attorneys after Judge Gerald Knight announced that the jury had not been able to reach a unanimous verdict.
As she was escorted from the courtroom in handcuffs, she said, "I want to thank my attorneys ... for doing a wonderful job."
Defense attorney Peter Mazzone called the penalty verdict bittersweet.
"It's sweet in the sense that we've been able to manage to stop another state-sponsored and state-financed killing of another human being," Mazzone told The Associated Press. "And it's bitter in that Barbara Opel will have to spend the rest of her life in prison."
Calls to the Snohomish County prosecutor's office were not returned Friday afternoon.
After the courtroom cleared out, jury spokeswoman Christine Wintch said, "We could sit back here for 99,000 years and we were not going to come to a unanimous agreement."
Opel was scheduled to be formally sentenced next Thursday.
All five youths who took part in the killing have been convicted or pleaded guilty.
Opel's daughter, Heather, and Marriam D. Oliver, 14 at the time of the killing, were sentenced to 22 years in prison last year after pleading guilty to first-degree murder. They each have appealed the decision to charge them as adults, and could see their sentences reduced if those appeals are successful.
Kyle Boston, 14 at the time of the slaying, was sentenced Wednesday to 18 years behind bars after pleading guilty to second-degree murder. Boston's cousin from Marysville, then 13, was convicted of first- and second-degree murder in juvenile court and can be held no longer than age 21.
Jeff Grote, 17 at the time of the attack, pleaded guilty to first-degree murder and likely faces a 50-year prison term. He is scheduled to be sentenced next week.
Greg Heimann, Jerry Heimann's son, said he had hoped Opel would get the death penalty.
"She's a very evil person," he said outside the courtroom. "She ruined a lot of lives - all those kids, our families - none of us will ever get over it."
The last time any prosecutors in Washington pushed for a woman's execution was in the case of Susan Kroll, who hired two Idaho men to shoot her husband in 1989 in Asotin County.
A jury found her guilty but couldn't reach a unanimous verdict on the death penalty, so she got life in prison without parole.