Janvier grew up in Tours, Indre-et-Loire, before joining a literary preparatory class at Jules Ferry High School in Paris.

She became a candidate in the 2017 French legislative election when she replaced the party's departmental point of contact (référent) Emmanuel Constantin when he was found ineligible due to his professional activity as a senior civil servant.

[8][9] In July 2019, Janvier published an information report on the European Union's plastics strategy, together with the Les Républicains MP Bernard Deflesselles.

[12] In July 2019, Janvier voted in favor of the French ratification of the European Union’s Comprehensive Economic and Trade Agreement (CETA) with Canada.

[13] In 2020, Janvier was one of ten LREM members who voted against her parliamentary group's majority and opposed a much discussed security bill drafted by her colleagues Alice Thourot and Jean-Michel Fauvergue that helps, among other measures, curtail the filming of police forces.