First term

Louijeune was elected to Boston City Council in November 2021. As a first-time candidate Louijeune had a strong showing in the 2021 election, finishing third in the at-large race behind incumbent council members Michael F. Flaherty and Julia Mejia.^[16][17] She is the first Haitian-American to serve on the council.^[16] Her election the city council was regarded as demonstrating their growing clout in the area's politics. According to U.S. Census Bureau data, Greater Boston is home to the third-largest Haitian diaspora population in the United States.^[18] She took office in January 2022.

In June 2022, the Boston City Council unanimously adopted a resolution introduced by Louijeune and Councilors Tania Fernandes Anderson and Kendra Lara which apologized for the city's historical role in the Atlantic slave trade.^[19]

In late 2022, Louijeune proposed an amendment to have the city regulate beekeeping.^[20]

In late 2022, Louijeune played a key role in the passage of a 20% pay increase for members of the Boston Council, which was vetoed by Mayor Michelle Wu. Wu supported an 11% increase, which had been the recommendation of Boston’s compensation advisory board, but opposed a 20% increase.^[21]

Louijeune and her City Council colleague Kendra Lara authored a resolution that was passed by the Boston City Council in late 2022 which urged Mayor Michelle Wu to raise the affordable housing unit requirements for new residential developments from 13% to 20% and to lower the threshold for which the requirements apply from buildings with nine or more units to buildings with five or more. The resolution also urged Wu to transition from utilizing HUD-designated area median income and to instead determine base affordability based upon the average income of a neighborhood.^[22]

In late 2022, Louijeune gave her support to the idea of permitting immigrants who have legal immigration status to cast votes in elections for city offices.^[23] More than 28% of Boston's city population are immigrants with legal immigration status. Fifteen other cities in the United States had already adopted similar measures. In December 2023, Louijeune voted to give City Council approval to a home rule petition that, if signed by the mayor, approved by the state legislature, and signed by the governor, would grant such voting rights in local elections.^[24]

After a judicial ruling required the city to adopt a new City Council district map to be used in the 2023 Boston City Council election, Ed Flynn, as president of the Boston City Council, tasked Councilor Louijeune with heading the process of drawing such a map in her capacity as the head of the Boston City Council's Civil Rights Committee. Flynn had assigned this task to Louijeune in order to avoid having Liz Breadon, the head of the Redistricting Committee, oversee it. The resulting map was adopted by the council in a 10–2 vote^[25] and signed into law by Mayor Wu.^[26] Louijeune's leadership in resolving the contentious redistricting matter raised her profile in the city's politics and won her praise.^[27]

In August 2023, Boston Herald political columnist Joe Battenfeld characterized Louijeune as having quickly become a "rising star" on the city council. He wrote that she had become, "a fast-moving leader of the body in less than two years."^[28] Other Boston political commentators had similarly called her a "rising star" on the council.^[29]

Second term and council presidency

Louijeune received the most votes in the at-large race of the 2023 Boston City Council election, being reelected to a second term.^[30] Two days after the election, Louijeune claimed that she believed she had secured enough support from fellow individuals elected to the incoming city council to be elected the council's next president.^[31] On January 1, 2024, after the new council was sworn-in, it voted unanimously to elect her as its president.^[32]