The City Council elected South Boston’s Bill Linehan as its president on Monday afternoon in an 8-to-5 vote that roiled progressive activists who supported a last-minute bid by Ayanna Pressley, a councillor at-large who lives in Dorchester. The vote potentially exposed new fault lines in the 13-member Council, which welcomed three new members and one familiar face to its ranks that same day.

The more progressive block did not have enough votes to elect Pressley and prevent Linehan, who was viewed as the conservative contender for the job, from ascending to the presidency. Aside from her own vote, she picked up support from District 4 Councillor Charles Yancey (Dorchester/Mattapan); District 6 Councillor Matt O’Malley (Jamaica Plain); District 7 Councillor Tito Jackson (Dorchester/Roxbury); and District 8 Councillor Josh Zakim (Back Bay/Mission Hill). Jackson and O’Malley, who had clashed with Linehan during last year’s battles over redrawing the nine Council districts, had each attempted a campaign for the top slot, but neither could get to the necessary seven-vote minimum.

The rest of the councillors threw in with Linehan, the councillor for District 2 since 2007. They included City Councillors At-Large Michelle Wu (South End), Stephen Murphy (Hyde Park) and Michael Flaherty (South Boston); District 1 Councillor Sal LaMattina (East Boston); District 3 Councillor Frank Baker (Dorchester); District 5 Councillor Tim McCarthy (Hyde Park); and District 9 Councillor Mark Ciommo (Allston/Brighton).

“The side that lost started a little late,” Baker said. “Billy had the votes.”

Reelected to his second term in November, Baker had initially been with Jackson and O’Malley. But when their coalitions fell apart, he decided for Linehan. He was apparently open to switching to Pressley if Wu would flip, but she had pledged her vote to Linehan and despite immense pressure from some progressive activists, she stood by her decision.

“I made a commitment and I’m looking forward to getting started,” said Wu, who voted for Linehan because he promised to decentralize the office of the president and put emphasis on the various committees.

Mayor Marty Walsh did not weigh in who should have the president’s chair, telling the Reporter last year that he had no interest in wading into the battle.

Michael Curry, head of the Boston branch of the NAACP, one of the groups that had lobbied against Linehan, said the District 2 councillor’s election as president was a “disappointing first vote” for the Council. His organization has pledged to keep a scorecard this year and plans to include the Linehan vote in its tabulation.

“There are all sorts of loyalties that came into play,” he said, noting that Pressley, in a speech on the floor just before the vote, argued that she would be a “citywide” Council president, contrasting herself with Linehan and his perceived parochialism. “You leave it on the floor and now it’s time to work with that City Council and that City Council president,” Curry added.

Linehan, after winning the gavel, struck a hopeful tone. “I will not let you, or Boston, down,” he said. He succeeds Murphy, who was term-limited in the presidency after three terms at the helm. The job is mostly ceremonial, but the president gets to hand out committee assignments and set the Council’s agenda. Linehan will also become acting mayor when Marty Walsh is out of town or otherwise unavailable.

Walsh has promised a collaborative approach in his dealings with the Council. But the city has a “strong mayor” system and the Council, despite often being stocked with rivals like Flaherty and former Councillor John Connolly, rarely overturned Mayor Thomas Menino’s vetoes or other decisions while he was in office.

On Monday, Walsh administered the oath of office to the new council at Conte Forum at Boston College after he was sworn in himself by Supreme Judicial Court Chief Justice Roderick Ireland. As they congratulated each other, Walsh said the councillors could take their time sitting back down, since he would not be telling them what they could and could not do.

“Not yet,” he added with a smile, drawing laughs from the crowd.