Rossi was first elected to the Commission scolaire de la Pointe-de-l'Île in 1998 and was re-elected in 2003 and 2007. She was not a candidate in the deferred election of 2014.
Borough councillor
Rossi was elected to the Montréal-Nord borough council in the 2009 Montreal municipal election as a Union Montreal candidate in Marie-Clarac. On council, she had special responsibility for culture and preparations for Montréal-Nord's centenary in 2015. She resigned from Union Montreal on February 21, 2013, after a corruption scandal implicated some of the party's leading members and joined Équipe Denis Coderre pour Montréal on June 17 of the same year.
City councillor
Rossi was elected to the Montreal city council for Marie-Clarac in the 2013 municipal election. Denis Coderre was elected as mayor and his party won a plurality of seats on council; on November 18, 2013, Coderre appointed Rossi as an associate councillor to the Montreal executive committee with responsibility for culture, heritage, and design. She worked in conjunction with Manon Gauthier, the executive councillor responsible for these files. Rossi continues to serve on the Montréal-Nord borough council by virtue of holding her position on city council. When Gilles Deguire resigned as borough mayor amid scandal in January 2016, Rossi was appointed as his interim replacement. She served in this role until April 2016, when Christine Black was elected to the position in a by-election. During her tenure as mayor, the city of Montreal introduced a sculpture resembling a ferris wheel as an identifying beacon in one of Montréal-Nord's busiest thoroughfares. The sculpture prompted some criticism for its $1.1 million cost, although many defended it on the grounds that it added a "rare bit of positive recognition" for the frequently troubled region of the city. Rossi was quoted as saying, "It's clear that Montreal North has experienced a lot of pain . This helps to put a balm on that pain." In the 2017 Montreal municipal election, Rossi ran as co-candidate for Coderre in the district of Ovide-Clermont. This means that if Coderre had been elected mayor and also city councillor for Ovide-Clermont, he would have sat as mayor and Rossi would have assumed the post of city councillor for Ovide-Clermont. Coderre won the city councillor election but lost the mayoral election; as he chose to resign from municipal politics, he exercised his option to cede the city council post to Rossi.
