Dennett co-founded The National Birth Control League in 1915 with Jesse Ashley and Clara Gruening Stillman. In 1918, she became the NBCL's executive secretary and started a campaign to make birth control information legal, giving lectures and lobbying state legislatures to change the laws. During this time, her pamphlet on sex education, "The Sex Side of Life," was published. Later, as the NBCL faltered, she resigned as executive secretary and founded a new organization, the Voluntary Parenthood League, which focused on repealing anti-birth control information laws at the federal level.