The merger was overseen by the CEO of Unicom, John Rowe, who joined the corporation in 1998 and led the newly formed Exelon until 2012, becoming the nation's longest-serving utility executive.

[7][8] On March 12, 2012, Exelon acquired Constellation Energy, with the combined company owning more than 34 gigawatts of power generation (55% nuclear, 24% natural gas, 8% renewable including hydro, 7% oil and 6% coal).

[11] The merger was originally rejected by the District of Columbia Public Service Commission in August 2015, though it was approved by other federal and state regulators.

[18] In 2005, Exelon was required to pay a $602,000 fine for exceeding the permitted sulfur dioxide emission limit from April to October 2004 at its Cromby Generating Station in Chester County, Pennsylvania.

Exelon and Illinois state officials waited for four years until 2006 before disclosing that Exelon's Braidwood Nuclear Generating Station, a nuclear plant 60 miles southwest of Chicago, had spilled millions of gallons of water containing tritium, a radioactive form of hydrogen, multiple times over a decade.

Exelon officials eventually apologized and said the risks from the leak were "minimal", with tritium levels in surrounding wells all found to be below regulatory limits.