Arias, who has said both that she wanted to live and wanted to die, looked visibly upset with the jury's decision. Before it was announced, she sobbed in the courtroom. Her family didn't attend Thursday but has been present for much of the trial.
In announcing the mistrial, Judge Sherry Stephens gave a heavy sigh and said: "This was not your typical trial. You were asked to perform some very difficult duties."
The jury began deliberating Tuesday and first reported it had failed to reach a unanimous decision the next day. Stephens instructed the jurors to keep trying.
The same panel on May 8 found Arias guilty of first-degree murder in the death of Alexander, who was stabbed and slashed nearly 30 times and nearly decapitated at his Mesa home. The jury later determined the killing was cruel enough to merit consideration of the death penalty.
Under Arizona law, a hung jury in the death penalty phase of a trial requires a new jury to be seated to decide the punishment. If the second jury cannot reach a unanimous decision, the judge would then sentence Arias to spend her entire life in prison or be eligible for release after 25 years. The judge cannot sentence Arias to death.
Former Maricopa County Attorney Rick Romley has said the case could drag on for several more months as the new jury reviews evidence and hears opening statements, closing arguments and witness testimony in a "Cliffs Notes" version of the trial.
However, if the prosecutor decides not to pursue the death penalty a second time, the judge would then sentence Arias to one of the life in prison options, and the trial would come to a conclusion.
The verdict came two days after Arias spoke directly to jurors and pleaded for her life. She said she "lacked perspective" when she told a local reporter after her conviction that she preferred execution to spending the rest of her days in jail. She told jurors she could bring about positive change in prison by teaching inmates how to read and helping launch prison recycling programs.
That night, Arias gave a series of media interviews from jail, telling reporters out about her many fights with her legal team and her belief that she "deserves a second chance at freedom someday."
Arias, 32, contends she killed Alexander in self-defense when he became enraged after a day of sex, forcing her to fight for her life. Prosecutors say she attacked him in a jealous rage because he wanted to end their relationship and go to Mexico with another woman.
Her case became a sensation from the beginning as Arias gave a series of jailhouse interviews following her 2008 arrest in which she blamed the killing on armed, masked intruders.
She went on trial in January, and the case provided endless amounts of cable TV and tabloid fodder, including a recorded phone sex call between Arias and the victim, nude photos, bloody crime-scene pictures and a defendant who described her life story in intimate detail over 18 days on the witness stand.
The former waitress told jurors of an abusive childhood, cheating boyfriends, dead-end jobs, her sexual relationship with Alexander, and her contention that he had grown physically violent.
The trial was streamed live on the Internet and became a real-life soap opera to people around the globe. Some even traveled to Phoenix to attend the trial and became fans of the fiery prosecutor, Juan Martinez, who repeatedly tangled with Arias during her testimony. They sought his autograph outside court and had pictures taken with him, prompting the defense to argue for a mistrial. Their request was denied.
The verdict brings a close to the trial's penalty phase, which featured dramatic statements by Alexander's sister and brother as they described how their lives were shattered by the loss of their beloved sibling.
Alexander, 30, overcame a tough upbringing in Southern California to become a successful businessman at a legal insurance company and a source of inspiration to his colleagues, his friends at his Mormon church and his family.
Arias stood confidently before the jury when she addressed the eight men and four women Tuesday, her voice breaking at times as she told them to spare her life for her family's sake. She said she planned to use her time in prison to design T-shirts that would raise money for victims of domestic abuse. She also said she would donate her hair to be made into wigs for cancer victims.
The judge had told jurors they could consider a handful of factors when deciding Arias' sentence, including the fact that she has no previous criminal record. They also could weigh defense assertions that Arias is a good friend and a talented artist.
Arias found it difficult to resist the spotlight throughout her case. She spoke to a Fox affiliate minutes after her conviction, and did a series of jailhouse interviews just hours after the jury got the case in the penalty phase.
"The prosecutor has accused me of wanting to be famous, which is not true," Arias told the AP on Tuesday in an interview where she combed her hair beforehand and wore makeup for the cameras. She also insisted that no images be transmitted of her from the waist down, showing her striped jail pants and shackled ankles.
Arias' attorneys tried without success to withdraw from the case several times during the trial, once after Arias gave her post-conviction TV interview, and another in the middle of her sentencing phase.