Skilling was found guilty on 20 counts of conspiracy, fraud, false statements and insider trading. He was found not guilty on eight counts of insider trading.
Lay was found guilty on all six counts of conspiracy and fraud.
In a separate bench trial, Judge Sim Lake ruled Lay was guilty of four counts of fraud and false statements.
Both Lay and Skilling could face 20 to 30 years in prison, legal experts say.