He will perform his duties “honourably, faithfully, impartially and conscientiously,’’ Khan promised before the judges in The Hague on Wednesday.
The representatives of the court’s 123 state parties had elected Khan in February.
The 51-year-old succeeds Fatou Bensouda, 60, who must step down after nine years under the court’s rules.
Khan wants to significantly improve the performance of the prosecution.
Proceedings must become more effective and trials before the court more successful, he said in a brief statement.
According to him, the criminal court is a sign of hope for justice.
“It is an awful testament of the horror of mankind in this 21 century, as we send rovers to Mars, as we are about to commence space tourism, where we managed to harvest the powers of the sun, wind, oceans.
“In this 21st century medieval crimes were committed by modern people: rape, killings, slavery, torture.’’
Khan is a highly experienced jurist in international criminal law.
The International Criminal Court is the only international court that prosecutes war crimes, crimes against humanity, and genocide.
The United States, Israel, Russia, and China do not recognise the court.
Khan takes office at a critical time for the court, which is about 20 years old.
It is under great political pressure because of planned investigations into war crimes in the Palestinian Territories and Afghanistan, which would also make trials against military officers from Israel or the U.S. possible.