The Democratic lawmaker said: “It is the moral thing to do to end the death penalty.”
Virginia is the first state in what was the Confederacy to abolish the use of the punishment.
Mr Northam signed the bill at the Greensville Correctional Centre near Garrett, where executions were previously carried out.
The state legislature passed bills ending the practice in late February. Virginia was one of the most prolific users of the penalty and has the longest history of implementing it, dating back to the first execution at Jamestown in 1608.
Governor Northam has criticised the death penalty as the “machinery of death.”
The state hasn’t executed anyone since 2017, or levied any capital sentences since 2011.
In February, Tim Kaine, US senator for Virginia, called the impending abolition in the “death penalty capital” a “singular achievement marking a repudiation of racism and a commitment to justice” in an op-ed in the Washington Post.
“Thankfully, the repeal of the death penalty by its leading practitioner gives hope that work for justice is not in vain,” he wrote. “Virginia’s progress shows that it is possible for all.”
Virginia is the 23rd state in the US to abolish the practice, and the first state in the South to do so, which experts say is a potent symbol, given the institution’s historic links to slavery.
Dating back to colonial times, the practice was grafted on to the systems of slavery and lasted through the Jim Crow era, where legal codes often specified harsher capital punishments for Black people. In modern times the penalty was disproportionately imposed on Black people.
“The death penalty is the direct descendant of lynching. It is state-sponsored racism and we have an opportunity to end this today,” said Democrat Jay Jones, speaking on the floor of the House in February.
The Biden administration has said it will “work to pass legislation to eliminate the death penalty at the federal level, and incentivise states to follow the federal government’s example”.
States in which execution remains on the books include ostensibly liberal bastions such as California and Oregon, as well as Georgia, Alabama, and Texas, where it is still deeply entrenched in the justice system.