[3] This left the Boston City Council president, at the time Kim Janey, to hold the role of acting mayor until the victor of the election would take office.

[4] On the morning of September 15, the counting of ballots reached 100% reporting with Michelle Wu as the first-place winner and Annissa Essaibi George in a second place.

In early 2021, incumbent mayor Marty Walsh was expected to resign to take the United States Secretary of Labor position.

Thus, if Walsh had left his position as mayor before then, a special election to fill the remainder of his term would have normally been required, per the city charter.

[10][11] The city council approved a home rule petition, which would dispense with the special election, on February 3;[12][13] it was subsequently signed by mayor Walsh.

[14] The petition next required approval from the state legislature (where it was filed as HD 1757, "An Act Relative to the Office of the Mayor of the City of Boston")[15] and governor.

[27] To appear on the ballot, candidates were required to file nomination papers at Boston City Hall by 5:00 p.m. on May 18 with 3,000 certified signatures of registered voters.

[78] Santiago withdrew from the race on July 13, with CommonWealth Magazine citing poor poll numbers and difficulty in building a field organization as his probable reasons for doing so.

[79] Writing on the primary election race, Ellen Barry of the New York Times called it "a departure" from the norm that the 2021 election has focused primarily on policy, rather than the candidates focusing on winning over particular racial/ethnic groups, remarking, "Boston's campaigns have long turned on ethnic rivalries, first between Anglo-Protestants and Irish Catholics, then drawing in racial minorities as those populations increased.

"[77] James Pindell of The Boston Globe wrote that some of the top topics debated in the primary were, "public schools, housing, development, policing, climate resiliency, drug usage, and mental health".

[80] Janey's campaign suffered a blow in early August when she expressed opposition to COVID-19 vaccine passports, likening them to slavery and birtherism.

[77] By early September, news sources largely considered Wu to have established herself in polls as the primary election's front-runner, with Andrea Campbell, Annissa Essaibi George, and Kim Janey being seeing as hotly contesting for a second-place finish.

Candidates are, by default, sorted in the table in the order of their total funds raised since launching their campaigns, from greatest (at top) to least (at bottom).

[151] With only a small fraction of the vote reported, Janey and Campbell conceded, and Wu and Essaibi George both gave victory speeches.

[160] The neighborhood of Hyde Park was considered a potential battleground in the election, due to it being home to a substantial voter base that had not backed either Wu or Essaibi George in the preliminary.