Technically, the first document to state this into law was the Ordinance of Villers-Cotterêts in 1539 (En, Fr), which replaced Latin with French as the required language of the courts and chanceries.

For the most part, there was high tolerance of regional languages (“patois” as the french derogatorily liked to call them) until about the revolution, and particularly starting with the second half of the 19th century (with the arrival of public education that strongly punished speaking anything but French). The major social and geographical movements of population of the two world wars are often credited with the final blow to most of the regional languages, few (if any) of which can still be called healthy.

I’m not entirely clear what the background in the last few decades was (I’m Canadian, But both English and French wikipedia have historical summaries of the French language policy), but a constitutional revision in June 1992 amended the French Constitution to enshrine French as the official language in Article 2 (Fr).