"This is a historic and momentous day for the people of Sierra Leone and the region," U.N. Secretary-General Ban Ki-moon said in a statement.
"The judgment is a significant milestone in international criminal justice, as it confirms the conviction of a former head of state for aiding, abetting and planning war crimes and crimes against humanity."
Stephen Rapp, the ambassador for war crimes issues at the U.S. Department of State and former prosecutor at the Sierra Leone court, said the ruling "sends a clear message to all the world, that when you commit crimes like this, it may not happen overnight, but there will be a day of reckoning."
However, it also appeared to establish dueling sets of jurisprudence at two international courts on opposite sides of The Hague on the question of when senior officials can support one side in another country's civil war — an issue world leaders must consider if they mull over arming rebels in Syria.
The Sierra Leone appeals panel rejected a controversial February ruling by the International Criminal Tribunal for the former Yugoslavia (ICTY), which said that to prove a leader has aided and abetted a crime, the assistance has to be specifically directed at committing a crime. In that case, the former chief of staff of the Yugoslav national army was acquitted of aiding and abetting atrocities by Bosnian Serb forces even though he had sent them arms and other supplies.
Taylor's case appeared to swing the pendulum back toward a lower burden of proof for prosecutors.
His lawyer complained that the two rulings have created "entirely chaotic jurisprudence" at international tribunals.
If Taylor had been prosecuted by the Yugoslav tribunal, "I dare say the outcome would have been different, and that courthouse is less than 10 kilometers (six miles) away from this courthouse," Morris Anyah said.
But international law expert Michael Scharf of Case Western Reserve University in Cleveland, Ohio, said the ruling Thursday "righted the ship" after the Yugoslav court had made prosecuting leaders who support rebels much more difficult.
Anyah also complained that Taylor had been prosecuted because of a lack of friends in high places, again referring to Syria.
"But for two powerful nations, two members of the Security Council — Russia and China — Bashar Assad would have been charged and indicted by the International Criminal Court. That is not happening simply because of political reasons," he said. "Had Charles Taylor had as friends any of the five permanent members of the Security Council ... this case I dare say would probably not have had the sort of traction it had."
Because Syria is not a member of the International Criminal Court, only the Security Council asking the court to investigate could trigger jurisdiction there.
Wearing a black suit and a gold-colored tie, Taylor showed little emotion while Presiding Judge George Gelaga King read the unanimous verdict of the six-judge panel.
Anyah said Taylor was bitterly disappointed by the ruling that will likely see him spend the rest of his life behind bars, but "he has remained stoic and calm."
It remains unclear where he will serve his sentence. Great Britain has offered cell space, but the court also has agreements with Finland, Sweden and Rwanda to house prisoners.