Mr Maimane was joined on stage by his wife Natalie as he received a standing ovation
South Africa's main opposition party, the Democratic Alliance (DA), has elected its first black leader, a move the party hopes will widen its appeal.
Mmusi Maimane, 34, was clear favourite to succeed outgoing leader Helen Zille.
Mr Maimane became party spokesman in 2011, regularly locking horns with the South African President, Jacob Zuma.
The DA won its best-ever share of the vote in the 2014 elections, but has struggled with perceptions it primarily represents white South Africans.
Mr Maimane faced party chairman Wilmot James for the job, and was elected at a party conference in Port Elizabeth.
He promised cheering supporters that the DA would win power and lashed out at Mr Zuma over corruption allegations.
"Make no mistake Mr President, you will have your day in court," he said in his victory speech.
Mr Maimane's supporters have been celebrating his election
Mr Maimane's predecessor, Helen Zille, made no secret of her belief that the party would never threaten the ANC's enduring grip on power if it could not finally shake off the accusation that it is a "white" party, representing the interests of those who have benefited from decades of racial apartheid.
To that end Ms Zille had tried to parachute in a prominent figure from the country's liberation struggle, Mamphele Ramphele, as the party's new leader before the last election. That plan collapsed in acrimony almost immediately.
Critics will now say that Mr Maimane's speedy ascent to the top job is another "parachute" job - that the 34-year-old is too young and inexperienced to deserve the position, and that his swift elevation smacks of racial tokenism.
And yet, anyone watching the crowds at the conference will know that the claim that it is remains a "white" party is increasingly at odds with the facts.
Mr Maimane has advanced quickly in the party, only joining the DA in 2009.
His first test will be taking on the ANC at next year's local elections. The party has governed South Africa since the end of apartheid.
"No party has a divine right to rule this country," said Mr Maimane in his speech.
He will also have to fend off a challenge from the hard-left Economic Freedom Fighters party, which threatens the DA's position as South Africa's main opposition party.
Ms Zille unexpectedly announced she would step down last month after increasing the share of the DA's vote in the 2014 poll.
She told the Citizen newspaper she was "relieved" to no longer be leader, but would continue to campaign for the party.
Mmusi Maimane is a recent member of the DA, joining in 2009