Jurors reached a decision late Wednesday morning after deliberating for about 15 hours over four days.
Arias gasped and appeared stunned when the guilty verdict was read. Inside the courtroom, some friends and family members cried and others hugged. Outside, a crowd of several hundred who came to the courthouse cheered when they heard the verdict.
The trial lasted four months, and Arias was on the stand for 18 days. The trial included graphic details of their sexual escapades and photos of Alexander just after his death.
Alexander, who grew up in Riverside County, suffered nearly 30 knife wounds, was shot in the forehead and had his throat slit. He was found by friends about five days later.
Authorities say the 32-year-old former waitress planned the June 2008 attack in a jealous rage after being rejected by Alexander while he pursued other women.
"She knew. She absolutely knew and had already planned it. She knew it; she was going to kill him," prosecutor Juan Martinez told jurors.
Arias initially denied involvement and later blamed the killing on masked intruders. Two years after her arrest, she said she killed Alexander in self-defense.
"I really thought he had intentions to kill me," Arias testified.
She said Alexander came at her "like a linebacker," body-slamming her to the tile floor. She testified that she managed to wriggle free and ran into his closet to retrieve a gun he kept on a shelf. She said she didn't remember stabbing Alexander. She acknowledged trying to clean the scene of the killing, dumping the gun in the desert and working on an alibi to avoid suspicion. She said she was too scared and ashamed to tell the truth.
The verdict ends one phase of the very long and very sensational case. Now, a second phase of the trial will take place, where the same jury will decide whether the killing was committed in an especially cruel, heinous or depraved manner. The outcome of this "aggravation phase" will affect how harsh the sentence could be, and Arias could get the death penalty.
Arias said in a post-conviction interview that she prefers a death sentence.
"I would much rather die sooner than later," she said.
The Associated Press contributed to this report.