His father became the president of GMAC (now Ally Financial) in Venezuela before he began his own business operations, and also was the Norwegian Crown's consul-general in Caracas from 1938.

[4] With his twin brother Olaf, he befriended Jerry Wolman, the owner of the Philadelphia Eagles football team, with the three becoming partners in real estate transactions and night clubs.

[4] Halvorssen’s activities were wide-ranging; he owned of Venezuelan insurance company AXXA Corretaje de Seguros and held representation positions at international conglomerates Dunlop, Ericsson, and British Cellophane.

[4] As chief of anti-narcotics, Halvorssen was behind the extradition to Italy of the Cuntrera-Caruana Mafia clan members Pasquale, Paolo and Gaspare Cuntrera.

[4] Halvorssen worked closely with Manhattan District Attorney Robert M. Morgenthau investigating the financial affairs of Venezuelan government officials.

"[10] According to Isabel Hilton, writing in the British edition of Gentleman's Quarterly (GQ), observers say that Pérez may even have offered Halvorssen the job as a necessary matching gesture when US president George H. W. Bush appointed William Bennett as his Director of National Drug Control Policy, thinking that Halvorssen was still drinking and would not raise issues.

[4][10] Halvorssen performed his duties well, deciding to use his worldwide contacts and intelligence connections to detail the extent of Venezuela's role in the drug world.

[11] During his time as commissioner, Halvorssen began an investigation with the assistance of New York District Attorney Robert Morgenthau, uncovering that President Pérez had a secret $19 million account with his mistress Cecilia Matos.

[4] The investigations by Halvorssen revealed that Castro was a business associate of Pablo Escobar, head of the Medellin drug cartel.

[3][11][12] During his time in prison, when there was an all out media campaign against him, Halvorssen was supported by the New York District Attorney Robert Morgenthau who praised his efforts as someone who provided assistance to his office in the fight against the international narcotics business, as well as to federal law enforcement officials of the United States.

[4] In March 1994, Halvorssen wrote a Wall Street Journal editorial recounting his ordeal and explained that prior to his arrest he had been investigating Banco Latino for money laundering and for being a Ponzi scheme.

Halvorssen explained that he was arrested in Caracas one day after a Banco Latino official asked him to come to Venezuela from New York for a meeting with the bank's chairman.

[citation needed] Wire reports and newspaper articles written during his time in prison portrayed Halvorssen as guilty and corrupt.

"[10] Armando Valladares describes Halvorssen's decision to serve as anti-Drug Czar as "quixotic" for having "simultaneously took on the entire banking sector without a budget or structural support in the government."

Orlando Castro Llanes fled Venezuela and was ultimately extradited from Miami to New York to face charges of grand larceny and theft.

[citation needed] Bestselling British investigative author David Yallop wrote a novel, Unholy Alliance, that included the story of his own involvement in Halvorssen's case.

At 36, Halvorssen became involved in community service, working at Venezuela's largest grant-giving charitable foundation, the Dividendo Voluntario Para La Comunidad for six years[4] and serving as its President from 1976–1979.

Halvorssen served on the boards of the Andrei Sakharov Institute and headed the Pan-American committee of the International Society for Human Rights.