The US district judge, Sim Lake, read out the guilty verdicts for Kenneth Lay, 64, and Jeffrey Skilling, 52, to a court in Houston, Texas.
Skilling was found guilty of 19 of the 28 charges against him and convicted of conspiracy, fraud, insider trading and making false statements which, combined, could mean a prison sentence of 185 years.
Lay was convicted of all six counts of conspiracy and fraud that he was charged with and could face 45 years in jail.
Skilling will remain free on a $5 million bail until sentencing, while Lake said Lay must post bail of $5 million and give up his passport to remain out of prison.
The judge said the two will be sentenced on September 11.
The verdict ends a four-year government effort to bring to justice those responsible for the collapse of Enron, an energy trading outfit that was once the seventh-largest company in the US.
The company folded in December 2001, after disclosures that it had used off-the-books deals to hide billions of dollars in debt and to inflate profits.
It was the biggest ever US bankruptcy at the time and investors lost billions.
It also transpired that the chief financial officer, Andy Fastow, had taken $60 million from the company while running the deals on the side.
Prosecutors said that Lay and Skilling knew Enron's reports of booming profits were talking up figures, but publicly declared all was well in order to keep the stock price up even as the power trader slid toward its demise.
Skilling could receive a
sentence of 185 years in prison
In testimony, Lay and Skilling said they painted a rosy picture of the company because they believed it was in great shape, not because they wanted to cover up problems.
Skilling suddenly resigned after just six months as chief executive in August 2001 and was replaced by the company chairman, Lay, who had been CEO before Skilling.
Prosecutors said the two men milked Enron for hundreds of million of dollars and lived lives of luxury while driving the company into bankruptcy.
John Hueston, assistant US attorney, said in his opening arguments, that Lay received $220 million in compensation from the sale of Enron shares from 1999 until 2001, while Skilling got $150 million.
Lay used his and the company's money to gain political power by donating heavily to candidates, particularly Republicans and especially the family of George Bush, the US president, who referred to him simply as Kenny Boy.
The convictions bring to 19 the number of Enron executives who pleaded guilty or have been found guilty of crimes.
Enron's demise raised questions about corporate oversight of executives in the US and was quickly followed by similar scandals at firms such as HealthSouth, WorldCom, Global Crossing and Adelphia.
After Enron, the US Congress scrambled to pass the Sarbanes-Oxley Act in 2002 toughening financial and auditing requirements for publicly owned companies.