Halvorsen was born in Norway in 1961. He graduated from the Norwegian Naval Academy and then served as a leader of a Norwegian SEAL team.
Halvorsen attended Williams College, where he was a member of the ski team, and graduated in 1986 with a degree in economics. He went on to receive his MBA from the Stanford University School of Business in 1990. At Stanford, he received the Alexander A. Robichek Student Achievement Award in Finance and was an Arjay Miller Scholar. Halvorsen is now on Williams' Board of Trustees.
In 1999, Halvorsen left Tiger to co-found Viking Global Investors, an investment firm managing in excess of $30 billion capital, of which he is currently the CEO, alongside David Ott and Brian Olson. Halvorsen, Ott, and Olson are part of the group known as "Tiger Cubs," former employees of Julian Robertson's Tiger Management Corporation now working as hedge fund managers on their own. Viking Global's main operations cover public equity and hedge funds on the international market.