Archival collections relating to the WILPF are held at the Unviersity of Colorado - Boulder. In looking through the online finding aids I did not see a list of Executive Directors but the information should be easy enough to track down in those materials. The library has several options for research assistance available. I would suggest contacting them via email and asking them (nicely) to look that info up, possibly in one of the annual reports series or historical publications. While their first responsibility is to the students of the University, most archivist take the responsibility of holding unique materials seriously, and are often glad of the opportunity to put them to use.
1968: Glenna Johnson began term as Executive Director
1970: Glenna Johnson ended term as Executive Director.
1971: Dorothy Steffens began term as Executive Director.
1977 (March): Dorothy Steffens ended term as Executive Director.
1977: Melva Mueller began her term as Executive Director.
1981: Libby Frank began term as Executive Director.
The previous historical summary section is not in timeline form, but includes this: "Mildred Scott Olmsted served as executive secretary of the Pennsylvania Branch from 1922 to 1946. Beginning in 1934, she also served as national organization secretary of the U.S. Section, with primary responsibility for maintaining close contact with branches and for developing branch work and organization. Her Organization Letter was an important channel of communication between the national officers and committee chairs and the state and local branch presidents and other members of the National Board. Olmsted became national administrative secretary in 1946. She retired as executive director in 1966 after 44 years of dedicated service to the Pennsylvania Branch and the U.S. Section."
It is not clear who served as Executive Director between Olmstead and Johnson, circa 1967.
Biographical information is not included in the finding aid.