The combined company will have about $16 billion in annual revenue and $1.4 billion in profit and serve more than 6 million customers in Pennsylvania, Ohio, Maryland, New Jersey, New York, Virginia and West Virginia.
"The combination of our companies is a natural fit that will accelerate our efforts to strengthen the operating performance of our generating fleet while building on our long-standing dedication to customers, shareholders and employees," Anthony J. Alexander, CEO of FirstEnergy, said in a statement.
Alexander will remain the top executive of the combined company, which will be based at FirstEnergy's headquarters in Akron, Ohio.
The deal is expected to close within 14 months.
Shareholders of Allegheny, based in Greensburg, Pa., will receive 0.667 shares of FirstEnergy common stock in exchange for each share they own. Based on Tuesday's closing price, those shares will be valued at $27.65 and represent a 32 percent premium over Allegheny's closing price of $21.02 on Wednesday.
Allegheny shares rose 19 percent to $25 in premarket trading.
The deal will also include FirstEnergy's assumption of $3.8 billion in debt.