Lagarde was born in Paris, France.

She graduated from Paris West University Nanterre La Défense, where she obtained master's degrees in English, labor law, and social law.

Lagarde joined Baker & McKenzie, a large Chicago-based international law firm, in 1981. She handled major antitrust and labor cases, was made partner after six years and was named head of the firm in Western Europe.

She joined the Executive Committee in 1995 and was elected the company's first female chairman in October 1999.

In 2004, Lagarde became President of the Global Strategic Committee.

As France's Trade Minister between 2005 and May 2007, Lagarde prioritized opening new markets for the country's products, focusing on the technology sector.

On 18 May 2007, she was moved to the Ministry of Agriculture as part of the government of François Fillon.

In 2010 Lagarde, then Finance Minister of France, sent a list of 1,991 names of Greek customers who were potential tax avoiders with bank accounts at HSBC's Geneva branch to the Greek government.

In July 2010, Lagarde told the PBS NewsHour that the IMF's lending program for distressed European countries was "a very massive plan, totally unexpected, totally counter-treaty, because it wasn't scheduled in the treaty that we should do a bailout program, as we did." She also said, "we had essentially a trillion dollars on the table to confront any market attack that would target any country, whether it's Greece, Spain, Portugal, or anybody within the eurozone."

On 25 May 2011, Lagarde announced her candidacy to be head of the IMF to succeed Dominique Strauss-Kahn, upon his resignation.

On 28 June 2011, the IMF board elected Lagarde as its next Managing Director and Chairman for a five-year term, starting on 5 July 2011.

On 3 August 2011, a French court ordered an investigation into Lagarde's role in a €403 million arbitration deal in favor of businessman Bernard Tapie.

On 28 October 2012, Greek reporter and editor Kostas Vaxevanis claimed to be in possession of the list and published a document with more than 2,000 names in his magazine Hot Doc.

On 20 March 2013, Lagarde's apartment in Paris was raided by French police as part of the investigation.

On 24 May 2013, after two days of questioning at the Court of Justice of the Republic (CJR), Lagarde was assigned the status of "assisted witness", meaning that she was not herself under investigation in the affair.

On 17 December 2015, the CJR (Court of Justice of the Republic) ordered Lagarde to stand trial before it for alleged negligence in handling the Tapie arbitration approval.

In December 2016, the court found Lagarde guilty of negligence, but declined to impose a penalty.

On 2 July 2019, Lagarde was nominated by the European Council to succeed Mario Draghi as President of the European Central Bank (ECB).

On 17 September 2019, the European Parliament voted via secret ballot to recommend her to the position, with 394 in favor, 206 opposed, and 49 abstentions. Confirmed by the European Council in its October summit, Lagarde became the first woman to serve as ECB President.