Right-wing parties, spearheaded by Sarkozy's UMP, took between 64 and 70 councils out of a possible 98, according to projections, as voters punished the socialist government of President Francois Hollande for failing to revive the slumping economy.
Left-wing parties took a beating, taking only between 27 and 37 councils, the projections by the Ipsos and CSA polling firms indicated.
"Never... Has our political family won so many councils," Sarkozy told cheering supporters, adding that voters had "massively rejected the policies of Francois Hollande and his government."
Pollsters said it was set to win between zero and two councils.
It would be the first time the FN has won any "departments", which control issues such as school and welfare budgets.
The party is often blocked from victory in second-round run-offs by tactical voting from mainstream voters, who gang up to keep the FN out of power.