Early results from 9,193 of 10,229 polling stations on Sunday showed Fillon leading with 67 percent votes and Alain Juppe in second place with 33 percent votes.
Fillon gave a victory speech soon after it became clear he had taken an unassailable lead. "What unites us is far more important than what differentiates us," he said.
He also saluted his opponent Juppe and former President Nicolas Sarkozy for backing him, and called President Francois Hollande’s five year in office as “pathetic”.
“France wants the truth and France wants action,” he said, adding: “Conservative voters have found in my campaign the values that are dear to them. I will defend those values, and we will share them with everyone that loves France.”
“I must now convince the whole country our project is the only one that can lift us up.”
He concluded by saying he wanted “a complete change in software” for France.
Fillon, as winner of the Les Republicains run-off, will compete in next year’s presidential election, where he could face the leader of the far-right Front National party Marine Le Pen.