"I Fatou Bensouda, solemnly undertake that I will perform my duties and exercise my powers as prosecutor of the International Criminal Court, honourably, faithfully, impartially and conscientiously," she said at a ceremony in The Hague.
The 51-year-old Bensouda, the first woman and African to head the team of prosecutors at the tribunal, had served as outgoing prosecutor Luis Moreno-Ocampo's number two since 2004.
Known as the public face of the ICC, Moreno-Ocampo stepped down Friday after nine years as chief prosecutor at the court, which started work in 2003.
Bensouda was elected by the 121 state parties that have signed up to the Rome Statute, the International Criminal Court's founding document.
She takes the helm of the world's first permanent court to try those accused of genocide, war crimes and crimes against humanity, investigating 15 cases in seven countries, all of them African.
ICC judges have issued 20 arrest warrants and nine summonses but only six suspects have been arrested so far and only one has been convicted, Congolese militia leader Thomas Lubanga who used child soldiers in the 2002-3 conflict in the northeastern Democratic Republic of Congo.
Those still on the run include Sudan's President Omar al-Bashir and Ugandan Lord's Resistance Army rebel leader Joseph Kony. Those arrested include Ivory Coast's toppled leader Laurent Gbagbo, who is awaiting a hearing to see if he will face trial on charges relating to violence that killed 3,000 people after the 2010 election.
To help her with meeting the demands of an expanding caseload, Bensouda and the court's states parties should make it a priority at the November meeting of all ICC members to elect a deputy prosecutor who has demonstrated excellence in dealing with complex criminal cases and institutional management.
There have been problematic past practices by the Office of the Prosecutor, Human Rights Watch said. For instance, in the Democratic Republic of Congo (DRC), Uganda, and Central African Republic (CAR) situations, the absence of charges against government officials without a clear explanation has undermined perceptions of the court's independence, Human Rights Watch said.
The perceived failure to pursue allegations against all sides in these countries has fed concerns that the prosecutor is yielding to pressure for "victor's justice," damaging the court's credibility.
The court's current investigations are in the DRC, Uganda, the CAR, Kenya, Côte d'Ivoire, the Darfur region of Sudan, and Libya, the latter two referred to the court by the United Nations Security Council.
Preliminary examinations concern Guinea, Colombia, Afghanistan, Georgia, Honduras, Nigeria, and South Korea - for acts committed by North Korea on South Korea's territory.