Before politics, she was president of the Fédération nationale des communications et de la culture [fr], Quebec's largest media union as well as a bassist in an all-lesbian alternative rock band, Mad June.

St. Onge also obtained a certificate in journalism from the Université de Montréal, and worked for La Presse in sales after graduation.

[3] In the early 2010s, St. Onge was the bassist, backup vocalist,[2] and manager of an all-lesbian alternative rock quartet from Montreal called Mad June.

By the 2019 federal election, St. Onge collaborated with Joly to bring most political parties to support such tax changes, which led to the later Online News Act.

[3] In the lead up to the September 20, 2021 federal election, Joly, the national campaign co-chair for the Liberal Party, recruited St. Onge, whose term at FNCC was ending.

[3][6] The count was subject to a judicial recount requested by the Bloc Québécois candidate after St. Onge won three days after election night on the strength of mail-in ballots.

[14] St. Onge refused to attend the 2022 FIFA World Cup in Qatar, citing the country's poor LGBTQ rights record as well as abuses of migrant workers.

[16] In February 2023, St. Onge organized a ministerial conference with provincial counterparts on the safe sport crisis in Charlottetown, P.E.I.

[19] In February 2024, St. Onge criticized Bell Media for a round of layoffs in local journalism positions and selling 45 of its 113 regional radio stations despite the end of certain licensing fees meant that it would gain $40 million in regulatory relief per year.