The panel of nine men and three women needed only two and a half hours to reach their "guilty" verdict, dashing defence hopes they would conclude that Smith had lost control of her actions and was guilty only of involuntary manslaughter when she sent her car rolling into a lake last 25 October, with her sons, three-year-old Michael and Alex, 14 months, strapped on to the back seat.
Only once did the jurors interrupt their deliberations, to ask to be shown once more the macabre television tapes of Smith tearfully appealing for her children's safe return in the days after she had drowned them, pretending they had been abducted by a black carjacker.
Afterwards her lawyer, David Bruck, played down the setback, saying both he and his 23-year-old client were anything but surprised. "She expected this, I expected this," Mr Bruck said. "For months Susan has been willing to plead guilty, so this is exactly what we thought would happen." Beyond question, however, the prospect of the first execution of a woman in South Carolina for half a century has moved closer.
As she listened to the verdict, the pale, bespectacled woman began shaking and had to be supported by her attorneys. Her distraught mother covered her face with her hands. Across the hushed courtroom, Smith's former husband David, who wants his former wife to die for killing their sons, watched impassively, a photo of his two dead children in his shirt pocket.
All now depends on the second, "penalty phase" of the trial, in which the jurors who convicted Smith will decide her punishment. The judge must follow their recommendation in handing down his sentence. But if the jurors do send her to the electric chair, South Carolina law stipulates their decision must be unanimous.
The 12-day trial has produced embarrassment and anguish enough for the close-knit town of 10,000 people, amid gothic tales of sexual molestation, adultery and incest. More is expected this week as the prosecution and defence produce more evidence.
"We are looking forward to the next stage," said Tommy Pope, the prosecutor, who has brushed aside many local misgivings over his relentless pursuit of the death penalty.