She will announce her candidacy on Monday.
In an interview Sunday, Harris, 28, said the firestorm over a controversial human resources report in 2019 that accused her of harassing the borough manager and a second civil servant influenced her decision to move from behind the scenes to the political stage.
Montreal Mayor Valérie Plante ousted Montgomery from Projet Montréal in January 2020 for refusing to fire Harris without seeing the full report.
In December, Quebec Superior Court sided with Montgomery, saying the city was unreasonable to ask her to fire Harris without her seeing the report or knowing the specifics of the allegations.