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.
In his victory speech, he called on the party to unite.
“We are democrats guided by our values. It is our values that unite us and I want to tell you today, it is our values that will lead us to victory.”
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 ascension 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.
Who is Mmusi Maimane?
• Born 1980, in the Soweto township of Johannesburg, where Nelson Mandela lived for many years
• Studied at universities in South Africa and Wales, holding Masters Degrees in theology and psychology
• Met his wife Natalie, a white woman, at church, telling iol.co.za “it took time to accept stereotypical responses”
• Speaks six languages
• Spends his weekends preaching in a Johannesburg church
• Worked in business consultancy before entering politics
• As DA parliamentary spokesman, he attacked President Zuma over his controversial Nkandla residence
Source: BBC News