Mildred Cox, 77, a former legal secretary and then a cashier at Leland Junior High School in Chevy Chase, died Thursday at Washington Hospital Center. She had a heart ailment.

Mrs. Cox was born in Virden, Manitoba, and attended business college in Calgary, Alberta.

After moving to Seattle as a young woman, she worked as a public stenographer and then a legal secretary for the corporation counsel of the city of Seattle.

In 1943, she married Earl Blake Cox, a former editor of the Masonic Tribune, a weekly newspaper in Seattle. Cox is now a retired Foreign Service officer. Before moving to this area in 1964, she accompanied her husband to diplomatic posts in Germany and Greece.

Mrs. Cox, who lived in Kenwood, worked as a cashier at Leland Junior High School from about 1964 until 1970. She was active in Concord-St. Andrews United Methodist Church in Bethesda.

Her first husband, Clarence Servin, a Seattle businessman, died in 1939.

Besides her husband, survivors include a son, Donald Servin of Waitsfield, Vt.; a daughter, Frances Ann Whittaker of Bellevue, Wash.; a sister, Glenna Synes of Portland, Ore., and six grandchildren.

The family suggest that expressions of sympathy be in the form of contributions to the choir fund at Concord-St. Andrews Church.