It was the first of several ovations for Mr. McDuffie, who told the lively audience he was “completely and utterly humbled” to assume the responsibilities of office.
He is charged with representing Ward 5 and restoring integrity to the seat vacated by Harry Thomas Jr., who resigned in January before pleading guilty to the theft of more than $350,000 in public funds. Thomas will report to a federal prison in Alabama on June 20 to begin a 38-month sentence.
“This moment is not lost on me,” he said. “I know where we are today, but I also know how far we have to go.”
Mr. McDuffie took the oath of office beside by his wife, Princess; his daughters, Kesi, 5, and Jozi, 2; and his parents, Allene and Gregory.
It was his mother, he said, “who taught me what it meant to serve others, and not just serve yourself.”
Mr. McDuffie, who worked as a letter carrier before he earned a law degree, graduated from Woodrow Wilson High School and still lives in the Stronghold neighborhood where he grew up. At 36, he is now the youngest council member.
He swept into city hall by securing nearly 45 percent of the ward vote in a special election May 15, prevailing over energetic competition from fellow Democrats Delano Hunter and Frank Wilds in a field of 11 candidates.
On his road to victory, Mr. McDuffie built a broad coalition of support from labor unions and council member Tommy Wells, Ward 6 Democrat, who cheered his stance on campaign finance reform.
“This election was about the residents of Ward 5. … The best days of the District of Columbia are ahead of us,” he told the crowded chamber on Wednesday.
His presence on the dais restores the council to its full complement of 13 members and provides a key swing vote, after a series of 6-6 votes in recent months put the council’s diminished numbers on full display.
The D.C. Board of Elections certified the election results at its meeting Wednesday morning and announced the official turnout as 17 percent of ward voters.
Before the campaign, Mr. McDuffie worked as a policy liaison in the office of Paul Quander, the deputy mayor for public safety. Before joining the Gray administration, he had been civil rights attorney for the U.S. Department of Justice and an assistant state’s attorney in Prince George’s County.
Until Wednesday, it had been more than a year since the council welcomed a new member. Vincent B. Orange, at-large Democrat, returned to the council in May 2011 after securing victory in a hotly contested special election.
Council Chairman Kwame R. Brown also directed Mr. McDuffie to his place on the council dais, placing his placard between Muriel Bowser, Ward 4 Democrat, and Jim Graham, Ward 1 Democrat.
“Council member Muriel Bowser is already cuttin’ deals,” Mr. Brown quipped, while Mr. McDuffie assumed his seat for the first time.