As is the case with other industries, ownership of nuclear power plants often changes over the years. Ownership of Three Mile Island has been no exception. At the time of the Three Mile Island Unit-2 Accident in March of 1979, General Public Utilities Corporation (GPU) was an electric utility holding company comprised of the following three operating companies: Jersey Central Power & Light Company (JCP&L), Pennsylvania Electric Company (Penelec), and Metropolitan Edison Company (Met-Ed). JCP&L operated wholly in New Jersey, and Penelec and Met-Ed operated in Pennsylvania at the time of the accident. The three operating companies were in joint-ownership of Three Mile Island Units 1 and 2 at the time of the accident, with Met-Ed as the direct operator of both plants. In January, 1982 operation of Three Mile Island Units 1 and 2 was divested from Metropolitan Edison Company and vested in General Public Utilities Nuclear Corporation (GPUN) – a newly created subsidiary of GPU – to disassociate the current actions of the company from those at the time of the accident.

In December 1999, General Public Utilities sold TMI Unit-1 to AmerGen Energy LLC, then a joint venture of PECO (now Exelon) and British Energy, of Edinburgh, Scotland. Exelon Corporation took full ownership of AmerGen in December 2003. On November 6, 2001, FirstEnergy of Akron Ohio closed its merger with General Public Utilities Corp., which ceased to exist as an electric utility holding company. Since then, JCP&L, Penelec, and Met-Ed have been subsidiary electric utility operating companies of FirstEnergy. As of 2004, TMI Unit-2 remains decommissioned and in post-defueling monitored storage. It is owned by FirstEnergy but maintained by Exelon, the owner and operator of TMI-1.

Plant Design
Plant design has generally been handled by third-parties such as Westinghouse Corp, General Electric, Combustion Engineering, and – as in the case of Three Mile Island Units 1 and 2 – Babcock and Wilcox. At the beginning of 2004 there were over thirty electric utility companies operating 104 electricity producing nuclear power reactors in the United States. The majority, if not all, of those reactors were designed and constructed by one of the four companies mentioned above. This does not include those nuclear reactors that are located at educational institutions and other organizations, which are used for research, training, medical and other purposes.