One of the few Black activists who were members of the Women's International League for Peace and Freedom (WILPF), McNeill joined the organization in 1934.

She led the organization to oppose lynching and Jim Crow laws and to adopt policies against scheduling meetings and congresses in segregated facilities while actively supporting school desegregation.

[1][2] She was the youngest child in the family which included three sons and four daughters – William, Luther, Oliver, Beulah, Mary, Elizabeth, and Bertha.

[1][3] After graduating, she moved to Washington, D.C., in 1905,[4] and furthered her education at Howard University, one of the Black colleges established shortly after the end of the American Civil War.

[1][4][6] On graduating from Howard in 1908 with a Bachelor of Arts degree,[1][7] McNeill began working at the Colored High and Training School in Baltimore, Maryland.

She also wrote articles for the Cape Fear Journal, an African-American newspaper which was founded by her sister Mary's husband, Robert S. Jervay, in 1927.

[2][14][15] She encouraged her nephew Robert H. McNeill, son of her brother William, to pursue his interest in photojournalism and bought him his first camera.

[21] As chair of the Committee on Public Affairs for the National Association of College Women, she formally wrote to the National Education Association to protest their selection of convention locations that would not allow Black attendees to lodge and required separate food, elevator, entrance, and seating arrangements.

[29] McNeill pointed out early in her tenure that while Black women were allowed to serve at the national level, in state and local organizations, they were often limited to membership of the Interracial Committee, rather than WILPF itself.

[32] Despite the internal conflicts, McNeill was committed to recruiting new Black members,[33] and in 1938 introduced Vera Chandler Foster, who organized a WILPF branch in Tuskegee, Alabama, and Lucy Diggs Slowe.

[36] McNeill served as a delegate from the United States to the Ninth International WILPF Congress held in Luhačovice, Czechoslovakia, in 1937.

[37][38] At the Congress, she brought up the case of the Scottsboro Boys, hoping to gain international support in linking peace initiatives with anti-lynching campaigns.

[40] Like many other Black peace activists, McNeill believed in diplomatic intervention as a means of preventing aggression and fascism, rather than a neutral stance.

The bill required women and aliens between the ages of eighteen and sixty-five to register for war assignments in either the civil or military service.

[21] Ultimately, none of the six bills proposed during World War II, which required women to register with the Selective Service System, passed into law.

[56] Writer Christine Lutz noted that McNeill and other Black activists in the 1950s, risked being charged with subversion for linking "peace with economic justice".