The former dictator was not without his supporters, who claim there was no genocide, but a high cost during a bloody civil war.
Human rights groups hailed the verdict.
"With this conviction, Guatemala leads by example in a region where entrenched impunity for past crimes sadly remains the norm," Sebastian Elgueta, Guatemala researcher at Amnesty International, said. "Guatemala must now follow up on this historic moment by ensuring that all those who took part in the murder, torture, rape and disappearance of tens of thousands of people are brought to justice."
The verdict also has implications for the United States' role in Guatemala during Rios Montt's rule.
When Rios Montt became president, human rights violations had already prompted the United States to cut off aid to the Guatemalan government. But a political scandal in the United States in the 1990s revealed that in fact the Central Intelligence Agency continued to provide money to Guatemalan military intelligence sources for years during the civil war.
Now-declassified secret CIA cables indicate the United States had knowledge of the atrocities being committed against the Ixil Mayans but did little about them.
Then-U.S. President Ronald Reagan went as far as to say that Rios Montt was being given a "bum rap" by critics. At the same time, the United States was backing other strongmen in Latin America against leftists.
Responding to Friday's verdict, the State Department noted the importance of a "strong and transparent justice system as an essential component of democracy," according to a statement from spokesman William Ostick.
"We see an opportunity for progress toward a true reconciliation in Guatemala, an essential step for the Guatemalan people to continue moving forward as everyone hopes," Ostick said.