Lewis, 34, pleaded guilty last month, admitting she had her husband, Julian Lewis, 51, and stepson, C.J. Lewis, 25, killed so she could collect insurance money. The men were shot to death Oct. 30 in their home in Keeling, about 120 miles southwest of Richmond.
Defense attorney David Furrow said he had expected Circuit Court Judge Charles Strauss would sentence Lewis to life in prison Tuesday.
One of the gunmen involved, 20-year-old Rodney Fuller, entered into a plea agreement in which he will get a life sentence if he testifies against the other alleged gunman, Matthew Shallenberger, 22.
Strauss said in court his decision was based on his interpretation of the law. He said Lewis appeared cold and emotionless throughout the proceedings, and she had no other motive for the murders except greed.
"She's crying," Furrow said of his client's reaction to the sentence. "She's going to die."
Pittsylvania County Commonwealth's Attorney David Grimes argued Monday that the risk Lewis posed to society and the amount of "planning and consideration" she put into the crime qualified her for the death penalty.
The defense maintained the plan was not primarily Lewis's.
Furrow said his client has an IQ of 72 according to tests conducted by a psychiatrist hired by the defense. A person with an IQ of 70 or lower is generally considered mentally retarded and ineligible for execution under a U.S. Supreme Court ruling last year.
The case now goes before the state Supreme Court for automatic review.
The last woman to be executed in the state was 17-year-old Virginia Christian in 1912, said Virginia Department of Corrections spokesman Larry Traylor. There are no records of women on death row since then, he said.
As of April 1 there were 48 women on death row in the United States, according to a report by the NAACP Legal Defense and Educational Fund, Inc.