The jury also found that Cynthia Sommer, 33, committed the first-degree murder with the special circumstances of poisoning and for financial gain.
Sommer, who stared forward as the verdict was read, faces life in prison without possibility of parole because of the special circumstances. She will be formally sentenced March 23.
In the court's spectator area, her mother burst into tears at the verdict and sobbed.
Prosecutors argued that Sommer wanted a more luxurious lifestyle than she could afford on the $1,700 monthly salary Sgt. Todd Sommer brought home and saw the military life insurance policy as a way to "set herself free."
"I'm deeply disappointed," defense attorney Robert Udell said after the verdict. "I don't believe Cindy killed Todd."
Of his client, he said, "She's quite stunned."
The seven women and five women of the jury chose not to talk to reporters after the verdict.
During the trial, Udell told the jury that the defendant had lost her "knight in shining armor," and repeatedly returned to the absence of any paper trail linking Sommer to the arsenic.
With no direct evidence that Sommer was the source of the arsenic detected in her husband's liver, Deputy District Attorney Laura Gunn relied heavily on circumstantial evidence of Sommer's financial debt to show that she had a motive to kill her 23-year-old husband.
Todd Sommer was in top condition when he collapsed and died at the couple's home on the Marine Corps' Miramar base in San Diego.
His death was initially ruled a heart attack. Tests of his liver later found levels of arsenic 1,020 times above normal.
Cynthia Sommer's friends and co-workers testified during the trial that she threw wild parties, got her breasts enlarged and had casual sex with multiple partners in the weeks after her husband's collapse.
Gunn asserted that the defendant was the only person with the motive and access to poison the Marine.
Todd Sommer spiked a 103-degree fever and visited an urgent care clinic on base complaining of gastrointestinal pain a week before his death on Feb. 18, 2002.
His widow testified that he had been well enough the day before to drink beer during a family trip to Knott's Berry Farm amusement park in Orange County.
Cynthia Sommer's in-laws testified that she objected when they asked her to put her husband's $250,000 death benefit in trust for herself, their baby and her three children from a previous marriage.
She cried when called to the stand Jan. 17, dabbing her eyes as she recounted her husband's final moments.
But she also said during cross-examination that she hadn't been able to envision her future with the Marine. The pair married in 1999.
She is now engaged to a former Marine she met two months after her husband's death. She was extradited last March to California from her current home in West Palm Beach, Fla.