FirstEnergy Corporation is a diversified energy company headquartered in Akron, Ohio. Its subsidiaries and affiliates are involved in the generation, transmission, and distribution of electricity, as well as energy management and other energy-related services. Its ten electric utility operating companies comprise the United States' largest investor-owned utility, based on serving 6 million customers within a 65,000-square-mile (170,000 km) area of Ohio, Pennsylvania, West Virginia, Virginia, Maryland, New Jersey and New York. Its generation subsidiaries control more than 23,000 megawatts of capacity, and its distribution lines span over 194,000 miles. In 2013, FirstEnergy ranked 181 on the Fortune 500 list of the largest public corporations in America.

In November 2016, FirstEnergy made the decision to exit the competitive power business, and become a fully regulated company during the next 18 months. It was anticipated that some generating units would be sold, and some would be shut down.

FirstEnergy was formed on November 7, 1997, when Ohio Edison acquired Centerior Energy and its subsidiaries for $1.6 billion in stock. The company was acquired with plans for a restructuring and layoffs to cut costs. That same month the Public Utilities Commission of Ohio (PUCO) initiated an investigation into the reliability of FirstEnergy's energy transmission in the context of possible plant shutdowns and prior problems with Centerior.

In 2001, FirstEnergy merged with GPU, Inc., the owner of Jersey Central Power & Light Company, Pennsylvania Electric Company (Penelec), and Metropolitan Edison Company (Met-Ed).

Ohio Edison Company (formerly OEC on the NYSE) was a publicly traded holding company that began in 1930, from the consolidation of 200 electric companies. By 1950, it ended up with two utility operating companies, Pennsylvania Power and Ohio Edison. It continued in existence until 1997, when its merger with Centerior formed FirstEnergy:

Centerior Energy Corporation (formerly CX on the NYSE) was formed in 1986 from the merger of two old operating companies. Centerior was based in Independence, Ohio, and existed as a publicly traded holding company for ten years, until its merger with Ohio Edison formed FirstEnergy in 1997:

General Public Utilities (formerly GPU on the NYSE) was a publicly traded utility holding company in Morristown, New Jersey. In 1996, the company was reorganized and renamed GPU, Inc. In 1996, it formed a new division as well, GPU Energy, which became the holding company for its three utility operating companies:

In 2001, FirstEnergy Corporation, with its four utility operating companies, merged with GPU, Inc., bringing GPU's three additional operating companies into FirstEnergy as well.

Through the 2001 acquisition of GPU, FirstEnergy also acquired:

GPU is best known as the former owner of the Three Mile Island nuclear plant. In 1989, Standley H. Hoch, a former executive with General Dynamics, became the CEO of GPU. Hoch had two main goals—cut costs and fight to repeal the Public Utility Holding Company Act of 1935, which made it difficult for utilities to operate across state lines.

Allegheny Energy was an electric utility serving customers in Pennsylvania, West Virginia, Virginia, and Maryland. Its regulated subsidiaries are West Penn Power (serving Southwestern and Central Pennsylvania), Monongahela Power (a.k.a. "Mon Power", serving Northern and Southern West Virginia), and The Potomac Edison Company (western Maryland, parts of eastern West Virginia, and northern Virginia). The electric generating plants are operated by subsidiary Allegheny Energy Supply Company and Monongahela Power.

Before the formation of Allegheny Energy, the holding company was known as Allegheny Power System which had the three utility operating units. The brand name Allegheny Power was used on customer bills, trucks and company equipment starting in 1996. In 1997, the company attempted to merge with Pittsburgh-based Duquesne Light Company. The merger was withdrawn by both parties and both companies did not merge. In 1999, Allegheny Power purchased the West Virginia operations of UtiliCorp United's West Virginia Power. UtiliCorp purchased Virginia Electric and Power Company's (present day Dominion Resources) West Virginia service area in 1986 and renamed the acquired service area West Virginia Power.

In February 2010, Allegheny Energy announced plans to merge with FirstEnergy. The merger was approved by stockholders of both companies, the Federal Energy Regulatory Commission, and regulatory commissions in Virginia, West Virginia, Maryland, and Pennsylvania. The merger was finalized when the Pennsylvania Public Utilities Commission approved the merger on February 24, 2011; the merger closed on February 25, 2011. The merger does not include Allegheny's service area in Virginia, which was purchased in 2010 by the Shenandoah Valley Electric Cooperative and the Rappahannock Electric Cooperative.

After the merger with Allegheny Energy, FirstEnergy was the largest investor-owned electric utility in the country for a short period of time based on customers served before the Exelon/Constellation and Duke Energy/Progress Energy mergers.

FirstEnergy announced its intent in 2016 to spin off its competitive businesses while staying in the regulated businesses. The FirstEnergy Solutions companies include unregulated portions of Ohio Edison, Toledo Edison, The Illuminating Company, Penelec, Met-Ed, Penn Power, West Penn Power, Jersey Central Power & Light (JCP&L), Mon Power, and Potomac Edison.

In August 2017, Robert E. Murray warned that FirstEnergy Solutions was in danger of bankruptcy if an energency order to open coal-fired plants was not issued by the White House. An emergency order to operate a coal-fired plant relieves the facility from compliance with environmental laws.

FirstEnergy's electric generation is primarily from coal and nuclear power plants. The system also includes natural gas, oil, and hydroelectric power plants. FirstEnergy operates the Beaver Valley, Davis-Besse, and Perry nuclear power plants.

A study conducted by the University of Massachusetts Amherst recognized the relatively low pollution profile of FirstEnergy. Although FirstEnergy placed 58 out of 100 of the country's largest polluters, the company performed better than its regional competitors, including American Electric Power, which ranked number 35.

FirstEnergy was required to pay $1.5 billion by 2011 as part of a settlement to end a lawsuit filed by the United States Environmental Protection Agency. This lawsuit alleged that the company failed to install pollution control equipment when upgrading its coal burning plants. Also as part of the settlement, major pollution control equipment is now being installed at the FirstEnergy Sammis site and others. This lawsuit was one of the New Source Review lawsuits filed in the 1990s.

To provide cleaner energy to its customers, FirstEnergy took several important steps in 2009. First, the company announced plans in April to repower units 4 and 5 at its R.E. Burger Plant in Shadyside, Ohio, to generate electricity principally with biomass, the only base load renewable source that can displace coal emissions. Furthermore, FirstEnergy is hosting a 1MWe pilot plant test of carbon capture retrofit equipment on one of the remaining coal units at the R.E. Burger Plant. In September, FirstEnergy decided to complete construction on the Fremont Energy Center, a 707-MW natural-gas-fired peaking plant by the end of 2010. And finally in November, FirstEnergy purchased the rights to develop a compressed-air electric generating plant in Norton, Ohio, which Ohio Governor Ted Strickland praised as "an example of how we can leverage technology and our natural resources to grow our economy and ensure our energy future." The Norton project, part of the company’s overall climate change strategy, has the potential to be expanded to up to 2,700 MW of capacity—the largest in the world by far. According to the Electric Power Research Institute, "a compressed-air energy storage project of this size...could be a key component in integrating large-scale intermittent renewables (such as wind) onto the nation's grid system." Together, these projects, when completed, will further reduce the utility’s emissions of CO₂, which already is about one-third below the regional average.

FirstEnergy Solutions Corp. has also given renewable energy certificates to help balance out the amount of electricity used in Earth Day events that were held at nine post-secondary education locations in Maryland, New Jersey, Ohio, and Pennsylvania. Each of the schools received five SmartWind REC’s, enough energy to light a large building for the entire day.

A July 2012 consent decree from the Pennsylvania Department of Environmental Protection is forcing FirstEnergy to close the Little Blue Run Lake, an unlined waste impoundment in Beaver County, Pennsylvania and Hancock County, West Virginia. Coal ash waste slurry has been piped there from FirstEnergy's Bruce Mansfield Power Plant since 1974. The reservoir at Little Blue Run is the country's largest coal ash impoundment. Pollutants including sulfates, chlorides, and arsenic have been found in groundwater nearby. FirstEnergy must stop dumping coal ash at the site by 2016, pay a penalty of $800,000, provide clean water to local residents, and do environmental monitoring of seeps for toxic pollutants including selenium, boron, and arsenic.

In November 2012 FirstEnergy led an aggressive behind the scene attack on Ohio's electric consumers. Quietly campaigning to have lawmakers amend state efficiency standards—without public hearings—during the lame duck General Assembly sessions. Energy efficiency became a state mandate in 2008 when lawmakers agreed with former Gov. Ted Strickland after months of debate to pass a law requiring electric utilities to help customers use less electricity every year—22 percent less by 2025 than they did in 2009. Under the 2008 law, FirstEnergy slaps companies that do not invest in energy efficiency with higher rates. But industries that use their waste heat to make power could escape that rate increase.

As of November 29, 2012 FirstEnergy Corp. has abandoned its behind-the-scene lobbying campaign to persuade lawmakers to gut a four-year-old law requiring utilities to help customers use less electricity by switching to energy efficient equipment and lighting.

Several cases have been brought against FirstEnergy for its dumping of coal waste into Little Blue Run Lake in West Virginia and Pennsylvania. FirstEnergy has dumped more than 20 billion gallons of coal ash and smokestack scrubber waste into the body of water which has contaminated local water supplies with arsenic, sulfates, sodium, calcium, magnesium and chloride.