In an emotional open-air ceremony, President Juan Manuel Santos welcomed the communist rebels into the political sphere after signing the accord with FARC leader Rodrigo Londono, alias Timoleon "Timochenko" Jimenez.
Dressed in white, the former mortal enemies signed and shook hands, smiling before an audience of international dignitaries, drawing loud cheers.
The ceremony in the Caribbean coast city of Cartagena followed a four-year process to end the last major armed conflict in the Americas. The accord remains to be ratified by referendum in a week.
"We are being reborn to launch a new era of reconciliation and of building peace," Timochenko said.
"In the name of the FARC, I sincerely apologize to all the victims of the conflict for any pain we may have caused during this war."
Colombian authorities estimate the territorial and ideological conflict has killed 260,000 people, left 45,000 missing and uprooted 6.9 million.
"Let no one doubt that we are moving towards politics without weapons. Let us all prepare to disarm hearts and minds," Timochenko said.
Santos then stepped up to the podium and addressed a message to the thousands of FARC fighters preparing to disarm in their jungle camps.
"When you begin your return to society... as head of state of the homeland that we all love, I welcome you to democracy," he said.
"Swapping bullets for votes and weapons for ideas is the bravest and most intelligent decision that any rebel group could take."
The 2,500 guests at the signing included UN Secretary General Ban Ki-moon, US Secretary of State John Kerry and the Vatican's Secretary of State Pietro Parolin.
An array of Latin American heads of state, including Cuban President Raul Castro, sat near the signatories on stage.
