Andres Pastrana was embraced by voters who spurned a key player in the scandal-tainted administration of President Ernesto Samper.
Pastrana, in his second straight bid for Colombia's highest office, had 50.9 percent of the vote with 89 percent of ballots counted, official results showed. Horacio Serpa of the governing Liberal Party had earned 46 percent of the vote so far.
"I believe that Colombia has really voted for a change," Pastrana, of the opposition Conservative Party, said in a television interview when results showed him ahead.
Public anger with Samper's tenure apparently helped Pastrana, who was narrowly defeated by the incumbent in 1994.
Serpa, a populist firebrand and veteran politician, was interior minister for most of Samper's administration, defending the incumbent in a corruption scandal that battered Colombia's international image and economy.
Pastrana launched his political career in 1982 when he became council of Bogota. In 1988, in the middle of his mayoral electoral campaign, he was kidnapped by the Medellin cartel.
Eight days later he was a free man, and his political career took off.
Pastrana now inherits a 34-year-old guerrilla war that has reached new depths of violence in recent months, with stepped-up massacres of civilians by right-wing paramilitary forces and assassinations of human rights workers.