His election to succeed current Mayor Shirley Franklin extends a decades-long line of black mayors in the mostly black city.
It came after a hard-fought race that was extended beyond November's general election with a December runoff and the recount. More than 84,000 ballots were cast in the runoff - more votes than in the general election.
The recount showed Reed won 714 more votes than City Councilwoman Mary Norwood. Norwood picked up just one vote in the recount. Because of the change, Fulton County elections officials said they would have to recertify the race on Thursday.
Norwood ran a strong grassroots campaign but fell short of becoming the city's first white mayor since 1973. She was scheduled to hold a news conference Wednesday afternoon.
The 40-year-old Reed, who entered the race a few months ago as a virtual unknown, ended the contest strongly, passing Norwood in fundraising. Reed began assuming his new role even as the Dec. 1 runoff was being sorted out, announcing his chief operating officer this week and an interim police chief as the recount began on Wednesday.
"The city of Atlanta has to have stability," said Reed, who takes office on Jan. 4. "These are decision that could not wait. We've got a lot to do. If we were to wait any longer, it would be Inauguration Day."
Reed and Norwood fought for votes across the city and over the airwaves in the month leading up to the runoff. Both tried to gain a critical mass of racial crossover votes, with victory likely hinging on black-versus-white turnout. Reed vowed Wednesday to continue to reach out to Norwood and her supporters as he prepares to take office.