Prosecutors argued that Cynthia Sommer, 33, wanted a more luxurious lifestyle than she could afford on her 23-year-old husband's $1,700 monthly salary and saw his military life insurance policy as a way to "set herself free."
Sommer's friends and co-workers also testified that she had casual sex with multiple partners in the weeks after her husband's collapse.
Sgt. 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 swallowed and stared as the verdict was read, while her mother burst into tears. She faces life in prison.
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 debts to show that she had a motive to kill her husband.
Gunn asserted that the defendant was the only person with the motive and access to poison the Marine.
The Marine's relatives 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.
Defense attorney Robert Udell told jurors that his client had lost her "knight in shining armor."
She is now engaged to a former Marine she met two months after her husband's death. She was extradited to California last March from her new home in West Palm Beach, Fla.