Mary H. Freeman, 67, a retired Montgomery County librarian who was active in conservation organizations, died of cancer Friday at Suburban Hospital in Bethesda.

A specialist on Early American decorating and tray-painting, she was a librarian in Bethesda and Rockville for 16 years before retiring from the Rockville library in 1975.

She also was active in the Friends of the National Zoo, the Audubon Society, the Wilderness Society, the American Museum of Natural History and the Animal Protection Institute of America.

Mrs. Freeman was born in Lima, Ohio, and grew up in Hyde Park, Mass. She attended the New England Conservatory of Music in Boston.

A resident of Rockville, she had lived in Montgomery County since 1950. She was a member of Christ Episcopal Church in Rockville, a resident associate of the Smithsonian Institution, and vice president of the Montgomery County Newcomb Club.

Her husband, James C. Freeman, a communications specialist for the National Security Agency, died in 1971.

Survivors include a daughter, Sheila Harvey, of Brookville, Md.; a son, James C. Jr., of Pittsfield, Mass.; three sisters, Kathleen Cairns, of Urbana, Ill., Jane Thompson, of Williamsburg, Va., and Sheila Ficken, of Waban, Mass., and four grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the American Cancer Society. CAPTION: Picture, MARY H. FREEMAN