Between 1956–1959 he was Senior Vice President of Raytheon, developing his management structure, allowing large degree of freedom for divisions while maintaining a 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 Europe it had telephone subsidiaries in numerous countries. In Brazil, it owned the telephone company. Washington feared that president João Goulart would nationalize it. Geneen was friends with the Director of Central Intelligence John McCone. The CIA performed psyops against Goulart, performed character assassination, pumped money into opposition groups, and enlisted the help of the Agency for International Development and the AFL-CIO. The 1964 Brazilian coup d'état exiled Goulart and the military dictatorship of Humberto de Alencar Castelo Branco took over. McCone went to work for ITT a few years later. The dictatorship lasted until 1985.
ITT also 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. Declassified documents released by the CIA in 2000 suggest that ITT financially helped opponents of Salvador Allende's government prepare a military coup. On September 28, 1973, an ITT building in New York City, New York, was bombed by the Weather Underground for alleged involvement in the September 11th overthrow of the Allende government.
In 1977 Geneen retired as CEO and president of ITT, was Chairman of the Board until 1979, and stayed on the board for four more years. His successors, particularly Rand Araskog, steadily sold off parts of the business.
In his obituary, The New York Times said that he remained active in business and on the boards of several Educational Institutes until his death, and had boasted that "his post-retirement deal-making had earned him far more than he ever made at ITT."
Geneen's widow, June Geneen, born in Berlin, New Hampshire, died in Boston in October, 2012.
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- The worst disease which can afflict executives in their work is not, as popularly supposed, alcoholism; it's egotism.
- You can't run a business or anything else on a theory.
- Leadership is practiced not so much in words as in attitude and in actions.
- The only unforgivable sin in business is to run out of cash.
- In business, words are words; explanations are explanations, promises are promises, but only performance is reality.
- In the business world, everyone is paid in two coins: Cash and Experience. Take the experience first. The cash will come later.
- Better a good decision quickly than the best decision too late.
- Telephones, hotels, insurance—it's all the same. If you know the numbers inside out, you know the company inside out.
- Leadership cannot really be taught, it can only be learned.
- I'd hate to spend the rest of my life trying to outwit an 18-inch fish. 
- A true leader has to have a genuine open-door policy so that his people are not afraid to approach him for any reason.
Harold Geneen wrote and co-authored several books:
- Geneen, Harold; Brent Bowers (1999). Synergy and Other Lies: Downsizing, Bureaucracy, and Corporate Culture Debunked. New York: St. Martin's Griffin. ISBN 0-312-20080-3.
- Geneen, Harold; Brent Bowers (1997). The Synergy Myth and Other Ailments Of Business Today. New York: St. Martin's Press. ISBN 0-312-14724-4.
- Geneen, Harold; Alvin Moscow (1986). Alta Dirección: Las Normas Básicas Para Triunfar en los Negocios. Barcelona: Ediciones Grijalbo. ISBN 84-253-1871-8.
- Geneen, Harold; Alvin Moscow (1984). Managing. Garden City, NY: Doubleday. ISBN 0-385-17496-9.
- Burn Before Reading, Admiral Stansfield Turner, 2005, Hyperion, pg. 99. Also see the article on Humberto de Alencar Castelo Branco. Also see BRAZIL MARKS 40th ANNIVERSARY OF MILITARY COUP, National Security Archive, George Washington University. Edited by Peter Kornbluh, 2004.
- Gilpin, Kenneth N. "Harold S. Geneen, 87, Dies" The New York Times, November 23, 1997
- Staff Report of the Select Committee To Study Governmental Operations With Respect to Intelligence Activities, December 18, 1975
- Hinchey Report at US Dept. of State
- Stout, David (January 30, 2003). "Edward Korry, 81, Is Dead; Falsely Tied to Chile Coup". The New York Times. Retrieved May 5, 2010.
- Montgomery, Paul L. (September 29, 1973). "I.T.T. OFFICE HERE DAMAGED BY BOMB; Caller Linked Explosion at Latin-American Section to 'Crimes in Chile' I.T.T. Latin-American Office on Madison Ave. Damaged by Bomb Fire in Rome Office Bombing on the Coast Rally the Opponents". The New York Times. Retrieved May 5, 2010.
- Ayers, Bill. Sing a Battle Song: The Revolutionary Poetry, Statements, and Communiques of The Weather Underground
- "Harold Geneen, 87; Led ITT's Growth for 18 Years". Los Angeles Times. November 23, 1997.
- Gilpin, Kenneth N. (November 23, 1997). "Harold S. Geneen, 87, Dies; Nurtured ITT". The New York Times.
- http://www.nytimes.com/1997/11/23/business/harold-s-geneen-87-dies-nurtured-itt.html?pagewanted=all (NY Times Obituary)