Mara R. Flaherty, 34, an associate with Porter, Novelli and Associates Inc., a Washington social marketing firm, who was active in drama circles, died of cancer Dec. 23 at Georgetown University Hospital.

Mrs. Flaherty joined Porter, Novelli in 1976, where she worked primarily with the National Cancer Institute's Office of Cancer Communications on its "Coping With Cancer" project. She was involved in the planning, writing and editing of several publications for the project, including "Coping With Cancer: A Resources for the Health Professional." In September, she addressed the Second International Conference on Cancer Nursing in London on the subject of "Living With Cancer."

She moved to Washington in 1974 and worked for the House subcommittee on the Environment, Interior, and Insular Affairs and for the Environmental Protection Agency before joining Porter, Novelli.

Born and reared in Iowa, Mrs. Flaherty graduated from Iowa State University in 1968 with a bachelor's degree in English and drama. That same year, she married Michael B. Flaherty, a naval aviator, and accompanied him to military assignments in California and Japan. Her marriage ended in divorce.

Mrs. Flaherty was a realtor in California and taught English, drama and women's studies at an American high school in Japan before coming here. A former student at the American Academy of Dramatic Arts in New York City, she appeared in summer stock productions in Iowa and acted in and directed community theater groups on American military bases in Japan. She also appeared in a 1975 production of the Chevy Chase Players.

Survivors include her parents, Mr. and Mrs. Donald Mogensen of Sergeant Bluff, Iowa, and a sister, Meg Cook of Dallas.

The family suggests that expressions of sympathy be in the form of contributions to the Vincent T. Lombardi Cancer Research Center, Georgetown University Hospital, Washington, D.C. 20007.