Anne Martin Johnston, 51, who was active in the Republican Party and in church, school and service organizations, was found dead Jan. 20 in the Potomac River near Great Falls National Park. The Montgomery County medical examiner said she had drowned and that the death was a suicide.

Montgomery County police said Mrs. Johnston had been missing since Jan. 14, when her car was discovered in the park.

Mrs. Johnston, who lived in Chevy Chase, had been a volunteer with President Reagan's White House transition team in 1980. She also had been a member of the Junior Board of the Davis Goodwill Memorial Industries, the needlework committee of the Washington Cathedral, various committees of the "Decorator Show House" for the National Symphony Orchestra, and the Junior League of Washington. She had also volunteered in various activities at the Landon School for Boys and the Stone Ridge School.

A native of New York, Mrs. Johnston attended the Foxcroft School in Middleburg, Va., and Bryn Mawr College. She moved to Washington in 1952 as a volunteer for President Eisenhower and later worked in the correspondence section of the White House.

Survivors include her husband, Ernest S. Johnston, whom she married in 1954, of Chevy Chase; four daughters, Katherine J. Camalier of Alexandria, Patricia J. Jermain of Washington, and Susan and Nancy Johnston, both of Chevy Chase; three sons, Ernest III, Robert and William, all of Chevy Chase; her father, H. Bradley Martin of New York; two sisters, Helen M. Bryan of Rumson, N.J., and Alice M. Tarafa of New York, and a brother, Bradley Martin of Marble Falls, Tex.