Hamm worked his way up from pumping gas and repairing cars to becoming CEO of his own billion-dollar company. As a young man, he went to work in the oil patch and in 1967 at the age of 21 he founded his own company, Continental Resources, a company that was originally called Shelly Dean Oil Company, named for his two daughters. Hamm and Continental Resources pioneered the development of the Bakken Oil Field in Montana and North Dakota using both horizontal drilled wells and Hydraulic fracturing. He became a billionaire and Continental Resources became a major oil producer. Hamm was given honorary degrees from Northwestern Oklahoma State University and the University of Oklahoma.

Hamm divorced his first wife, Judith Ann, in 1987. They had three children together.

In April 1988, Hamm married Sue Ann Hamm (née Arnall) with whom he has two grown daughters, Jane and Hillary. Sue Ann is an economist and lawyer. She has had executive roles at Continental Resources. She filed for divorce on May 19, 2012, while Harold has said that he separated from Sue Ann in 2005. In 2014 Forbes reported that a ruling would allow the bulk of his ownership interest in Continental—and hence his net worth—to continue intact after the divorce. However, in August 2014, CNN and several other media outlets reported that Hamm could face a world record divorce settlement, with up to half of his estimated $20 billion fortune potentially being on the line. On November 10, 2014, an Oklahoma County judge found that Hamm should pay his second wife $995.5 million. According to an article published on January 7, 2015 by the Wall Street Journal, “She rejected a divorce settlement check in the amount of $974,790,317.77, stating it would jeopardize her appeal with Mr. Hamm.” On January 8, 2015, CNBC reported that Sue Ann Arnall had deposited the $975 million settlement check.

Hamm is a key player in Hiland Partners and Hiland Holdings as well as the shale oil-exploration company Continental Resources, Oklahoma’s fourth largest public company. Continental Resources moved from the Continental Towers in Enid, Oklahoma, to Oklahoma City in 2012. Hamm was inducted into the Oklahoma Hall of Fame in 2011.

Shortly after being named energy advisor to the Romney campaign in March 2012, Hamm donated $985,000 to the pro-Romney super PAC Restore Our Future. On the eve of the Republican National Convention, Hamm was credited in a report with being among oil and energy executives consulted in a plan to devolve permitting on Federal lands to the state where the land is located. Romney’s proposal, which was launched the same week as two industry fundraisers netted $10 million for the campaign, was seen as going beyond roots in the Sagebrush Rebellion of the 1970s and 1980s, as well as the sometimes controversial permitting policies of the George W. Bush Administration in the 2000s.

Hamm appeared on the cover of Forbes in May 2014 in a story entitled “Harold Hamm: The Billionaire Oilman Fueling America’s Recovery”.

On the third day of the 2016 Republican National Convention, held in Cleveland, Ohio, on July 20, while delivering a speech for the then Republican presidential nominee Donald Trump, Hamm was critical of the Barack Obama administration’s policies for the energy sector, citing the burden that federal regulations represented to American oil producers in an alleged attempt to drive gasoline prices soaring. In his speech, Harold Hamm also denounced the Iran nuclear deal struck by the U.S. Department of State in 2015, saying that it would improve that country’s ability to export petroleum and enable Iranian development of the atomic bomb. Hamm also spoke favourably of Trump’s policies for the oil and gas industry, which he claimed, if they were implemented, would make Donald Trump the first President to guarantee America’s “energy independence”. In that same month, it was revealed that Donald Trump was considering Harold Hamm for energy secretary in his administration.

On May 15, 2015, Bloomberg reported that Hamm told Larry Grillot (the dean of the University of Oklahoma Mewbourne College of Earth and Energy) that he wanted certain earthquake researchers from the university dismissed. The scientists were involved in studying connections between oil and gas activity and the increase in the earthquake activities in the state.

In January 2016, Hamm stated that Saudi Arabia’s attempt to flood the crude market at a time of oversupply and concerns about weakening demand is not working, believing that oil prices will double by year-end 2016, as Shawn Baldwin, Chairman of alternative investment firm The AIA Group, concurred with these statements.