Taylor has been on trial at the UN-backed Special Court for Sierra Leone for almost five years.
He was accused of backing rebels who killed tens of thousands during Sierra Leone’s 1991-2002 civil war.
But he was cleared of ordering their crimes.
Taylor is the first former head of state convicted by an international court since the Nuremburg military tribunal of Nazis after World War II.
Human rights groups described the judgement as historic.
“This is an incredibly significant decision,” Elise Keppler from the campaign group Human Rights Watch told the BBC. “Today is a landmark moment.”
Rights group Amnesty International said the verdict sent an important message to all high-ranking state officials.
“While today’s conviction brings some measure of justice to the people of Sierra Leone, Taylor and the others sentenced by the Special Court are just the tip of the iceberg,” the group’s Brima Abdulai Sheriff said in a statement.
Judge Richard Lussick said Taylor had sold diamonds and bought weapons on behalf of the Revolutionary United Front (RUF) rebels – and knew they were committing crimes.
“The chamber finds beyond reasonable doubt that the accused is criminally responsible… for aiding and abetting the commission of the crimes 1 to 11 in the indictment,” judge Lussick told the Special Court for Sierra Leone, sitting in the outskirts of The Hague, as he read the verdict.
The accused had substantial influence over the RUF, but this fell short of effective command and control, the judge said.
The military support provided by the accused to the RUF had a significant impact on the commission of crimes, the judge added.
Judge Lussick also said the prosecution failed to prove beyond reasonable doubt that the accused was individually responsible for some of the crimes committed in Sierra Leone or part of a joint criminal enterprise.
A sentence hearing will be held on 16 May, with the sentence to be handed down on 30 May, he added.
Taylor is expected to serve his sentence in a British prison as the Dutch government only agreed to host the trial if any ensuing jail term was served in another country.