Catherine Griset, Le Pen's office chief, was charged with breach of trust in a probe into allegations the candidate's National Front (FN) party defrauded the European Parliament of about 340,000 euros ($360,000).
Griset and Le Pen's bodyguard Thierry Legier were interviewed by anti-corruption police on Wednesday, but Legier has not been charged.
The European Parliament accuses Le Pen, a deputy in the chamber, of using parliamentary funds to pay the two employees while they worked for the FN in France rather than at the Parliament's premises.
Le Pen has refused to repay the funds and the European Parliament will begin clawing back the money next month by docking it from her monthly parliamentary salary and allowances of 16,000 euros.
The FN leader has angrily denied the allegations, and on Wednesday again described the investigation as a vendetta against her.
Despite the allegations, opinion polls currently show her winning the first round of the election in April, although she is forecast to lose in the runoff in May.
© 2017 AFP