She told congress-goers the DA would continue fighting against the ANC in her maiden speech as leader.
"The only idea [the ANC] have left is race… The DA must provide the political and moral leadership the country needs."
Zille also said the DA would be willing to build governing coalitions with like-minded parties.
She first came to public attention when working as a journalist 30 years ago, exposing events behind the killing of Steve Biko, a black rights activist.
"Our biggest challenge in this country is the politics of identity... to break away from the shackles of race"
Helen Zille, new leader of the Democratic Alliance
She later became a provincial education minister and her successive experience as Cape Town mayor has earned her respect as an administrator.
Her DA leadership campaign was focused on providing the organisation with appeal across South Africa's racial spectrum.
"Our biggest challenge in this country is the politics of identity... to break away from the shackles of race," Zille said as she announced her candidacy last month.
The DA, which has 57 parliamentary seats out of 400, has a very small share of the black vote.
Analysts say the change of leader is an opportunity for the DA to shake off its white elitist image. The ANC has regularly accused Leon of being elitist and racist.
Seremane, who has a political background steeped in the anti-apartheid struggle, said the party should become more representative during his leadership campaign.