Frances E. Willis, 84, the first woman career officer in the State Department Foreign Service to serve as an ambassador, died July 20 at her home in Redlands, Calif., after a stroke.

Miss Willis was commissioned in the Foreign Service in 1927. Her first assignment was as a consular officer in Valparaiso, Chile. She retired in 1964 at the end of a career in which she had been ambassador to Switzerland, Norway and Ceylon, now Sri Lanka. She held the personal rank of minister counselor in the Foreign Service.

It was Miss Willis' distinction that she made her way to ambassadorial posts entirely on the career ladder. Other women, such as Eugenie Anderson, Clare Booth Luce and Perle Mesta, headed U.S. diplomatic missions abroad before the honor came to Miss Willis. But they were appointed because of political considerations rather than their professional qualifications.

Miss Willis was named to head the embassy in Switzerland by President Dwight D. Eisenhower in 1953. Previously, the top American diplomat in that country had held the rank of minister. In 1957, Eisenhower named her ambassador to Norway. President John F. Kennedy sent her to Ceylon in 1961 and she retired from there in 1964.

In an interview in 1953, Miss Willis said that qualifications for a career in the Foreign Service can vary, but that any aspirant requires the "necessary mental equipment" and the ability to "deal effectively with other people" and be "an accurate reporter."

Her own mental equipment included bachelor's and doctoral degrees in political science from Stanford University and a year of study at the University of Brussels. She taught at Goucher and Vassar colleges before joining the Foreign Service. She was fluent in French and Spanish and could read German.

After Valparaiso, her posts included Santiago, Stockholm, Brussels and Luxembourg. She spent much of World War II as first secretary and consul of the U.S. Embassy in Madrid. She later was in the State Department as an assistant to the secretary and assistant head of the division of European affairs.

In 1947, she went to London as first secretary of the embassy. In 1951, she was posted to Finland as counselor of legation and deputy chief of mission. She remained there until her appointment to Switzerland.

Miss Willis was born in Metropolis, Ill., but she regarded Redlands as her home town and returned there when she retired.

Survivors include a brother, Henry, of Salinas, Calif.