The insurer said Benmosche is undergoing aggressive chemotherapy. He said in a statement his long-term prognosis is good and he continues to work his normal schedule. He said he will know more about the course of his treatment in coming weeks as his doctors analyze the results of his chemotherapy.
The company announced Benmosche's diagnosis after the market closed Monday.
Its stock fell 26 cents, or nearly 1 percent, to $40.84 during aftermarket trading.
Robert Miller, chairman of AIG's board of directors, said the company will engage in contingency planning to ensure management continuity as Benmosche's treatment unfold.
Benmosche has led AIG's efforts to repay a $182 billion government bailout that pulled the company from the brink of bankruptcy in 2008. In September, AIG unveiled a plan to pay off the bailout. Under the deal, Treasury will swap its majority stake in AIG for common stock and then sell those shares over time.
Benmosche said in the statement Monday that repaying the bailout is still his top priority.
"AIG is executing on an established, clear roadmap to repay taxpayers. Just in the last week we have made extraordinary progress on a key asset divestiture," he said.