Mills had been set to run for re-election in the December vote before he died on July 24 at age 68. No official cause has been given, but there have been unconfirmed reports that he suffered from throat cancer.
Some 2,000 delegates, including Mahama and former president Jerry Rawlings, were attending the congress in Kumasi in south-central Ghana, traditionally a stronghold of the opposition but where the NDC is seeking to make inroads.
Mahama, who had been vice president before Mills’s death, was sworn in to serve out the remainder of the late leader’s term, as dictated by the west African nation’s constitution.
Mills’s death upended the presidential race in a country that recently became a significant oil producer and is praised as a stable democracy in an often turbulent region. The transition has so far gone smoothly.
Analysts say the election is likely to be close. Mills won the 2008 vote with less than a one percent margin