Geneen was born on January 22, 1910 in Bournemouth, Hampshire, England and migrated to the United States as an infant with his parents. He studied accounting at New York University.

Between 1956—1959 he was Senior Vice President of Raytheon, developing his management structure, allowing large degree of freedom for divisions maintaining high degree of financial and other accountability.

From 1959—1977 he was the president and CEO of International Telephone and Telegraph Corp. (ITT). He grew the company from a medium-sized business with $765 million sales in 1961 into multinational conglomerate with $17 billion sales in 1970. He extended its interests from manufacturing of telegraph equipment into insurance, hotels, real estate management and other areas. Under Geneen's management, ITT became the archetypal modern multinational conglomerate. ITT grew primarily through a series of approximately 350 acquisitions and mergers in 80 countries. Some of the largest of these were Hartford Fire Insurance Company (1970) and Sheraton Hotels.

ITT had many overseas interests. In particular ITT had some $200 million-worth of investments in Chile. Under Geneen's leadership, ITT funneled $350,000 to Allende's opponent, Jorge Alessandri. When Allende won the presidential election, ITT offered the CIA $1,000,000 to defeat Allende, though the offer was rejected.

In 1977 Geneen retired as CEO and president of ITT, staying on as Chairman of the Board until 1979. His successors, starting with Rand Araskog, steadily sold off parts of the business.

Harold Geneen wrote and co-authored several books: