[citation needed] After university Kellogg worked for Charities magazine before carrying out an unprecedented, in-depth study of industrial life in Pittsburgh.
An opponent of U.S. involvement in the First World War, Kellogg joined Jane Addams and Oswald Garrison Villard, to persuade Henry Ford, the American industrialist, to organize a peace conference in Stockholm.
The Ford Peace Ship reached Stockholm in January, 1916, and a conference was organized with representatives from Denmark, the Netherlands, Norway, Sweden and the United States.
In 1920, Kellogg joined with Roger Baldwin, Norman Thomas, Crystal Eastman, Addams, Clarence Darrow, John Dewey, Abraham Muste, Elizabeth Gurley Flynn and Upton Sinclair to form the American Civil Liberties Union.
In 1927, Kellogg joined with John Dos Passos, Alice Hamilton, Addams, Upton Sinclair, Dorothy Parker, Ben Shahn, Edna St. Vincent Millay, Floyd Dell, George Bernard Shaw and H. G. Wells in an effort to prevent the execution of Nicola Sacco and Bartolomeo Vanzetti.