She is succeeding her father Jean-Marie Le Pen, who founded the FN in 1972.
On Saturday party sources reported that she had secured two-thirds of votes against rival Bruno Gollnisch in a recent referendum of members.
The anti-immigrant FN has been shunned by France's main parties, but Ms Le Pen has said she wants to break with the party's xenophobic image.
In a combative farewell speech on Saturday Mr Le Pen, 82, insisted that "unceasing immigration" posed a threat to France.
"All my comments were distorted from their true meaning... because I refused to submit to the dictatorship of the thought police," he told cheering supporters at the conference in the central city of Tours.
He added that it was up to FN members to ensure the party's future success under a new leader.
"I entrust you with the destiny of our movement... its unity, its pugnacity," he said.
French TV footage showed Marine Le Pen, 42, crying as she applauded her father.
Although Mr Le Pen's five presidential bids have failed, the FN has steadily grown under his leadership. In recent elections the party has been able to garner about 15% of the vote.
In 2002 he came a shock second in the first round of presidential elections, but lost the second round to incumbent Jacques Chirac.
A recent poll suggested the party could come third in presidential elections to be held in 2012.
BBC News
