PUTRAJAYA, Malaysia—Jho Low, the alleged brains behind one of the world’s largest financial scandals in recent years, is officially now a fugitive after Malaysia’s Prime Minister Mahathir Mohamad said investigators are seeking his arrest.

The U.S. Justice Department alleges Mr. Low was the architect of a massive fraud encompassing the 1Malaysia Development Bhd. sovereign-wealth fund. From 2009 to 2015, at least $4.5 billion was siphoned from its accounts. The Justice Department’s civil suits say he paid off conspirators around...