Patrick J. McCarthy, 66 a retired economist with the Federal Energy Regulatory Commission who was active in civic affairs, died of cancer Thursday at Georgetown University Hospital.

Mr. McCarthy was born in Staten Island, N.Y. He became a lieutenant in the New York City Fire Deparment and earned a bachelor's degree in business administration at New York University. He also did graduate work there.

In 1957, he moved to this area and joined the old Federal Power Commission. While working for the government, he earned a master's degree in marketing from American University. At his retirement in 1977, Mr. McCarthy was an official of the Federal Energy Regulatory Commission, which had taken over many of the rate-setting and other functions of the FPC.

Mr. McCarthy was a past president of the Baileys Crossroads Lions Club and a member of the board of the Lake Barcroft Community Association. (SECTION) urvivors include his wife, Doris A., of the home in Falls Church; two daughters, Megan D., also of the home, and Laurel a. Collins, of Fall River, Mass.; a sister, Mary Fredericks, of Morristown, N.J., and two grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the American Cancer Society. CAPTION: Picture, PATRICK J. McCARTHY