Dennett was a precocious, talkative, and assertive child, "scolding [her older brother] for striking her, often quoting the Bible."

While her mother was absent on tours, Dennett and her siblings often lived with their Aunt Lucia Ames Mead, a prominent social reformer.

In addition to her work as an interior designer and guadamacile[9] maker, Dennett continued to lecture and write about the Arts and Crafts movement.

This time the doctor told the Dennetts that they should not have any more children, due to a laceration in her uterus that required corrective surgery.

[18] Dennett co-founded the Twilight Sleep Association (1913), which advocated the use of scopolamine and morphine to ease the pain of childbirth.

[21] Dennett's work to re-elect Woodrow Wilson (under the belief that he would not declare war) led to a respected job as executive secretary for the League for Progressive Democracy.

Dennett feared the negative effect that her involuntary notoriety might have on the organizations she worked with and considered resigning from the Twilight Sleep Association.

[27] Dennett co-founded The National Birth Control League in 1915 with Jesse Ashley and Clara Gruening Stillman.

[28] 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.

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.

"Mary Dennett sounded defiant, proclaiming she would pay no fine, however small: 'If a few federal officials want to use their power to penalize me for my work for the young people of this country, they must bear the shame of a jail sentence.

She began as field secretary of the Massachusetts Suffrage Association, organizing lectures, rallies, sermons, cheap meals, speaking tours to gather signatures for petitions, and similar outreach efforts.

"[30] In 1910, Dennett's success in Massachusetts led the National American Woman Suffrage Association to aggressively recruit her for the position of Corresponding Secretary, reporting to Dr. Anna Howard Shaw.

[31] Later, Dennett became disillusioned with NAWSA after an unsuccessful attempt to reorganize to be more effective and what she saw as wasteful decisions overly influenced by wealthy donors.

[27] Dennett decided to start by rallying public support to strike down laws restricting birth control information.

Beginning in 1919, Dennett focused on a "straight repeal" of the birth control provisions of the Comstock Act at the federal level, rather than state-by-state efforts.

[40] Dennett achieved her goal in an entirely different manner in 1930, by winning an appeal of her conviction for distribution of birth control information under the Comstock Act.

Many existing sexual-education publications either contained inaccurate information or used fear and shame tactics to dissuade the youth from having sex.

The pamphlet covered controversial topics including masturbation, sexually transmitted diseases, prostitution, and support for the use of birth control.

After fours years of being in circulation, the Post Office informed Dennett that the pamphlet was obscene, and therefore it was banned from being mailed under the Comstock Act.

He had briefly praised Mrs. Dennett's book [42] in the May 1926 issue of The American Mercury, and took a sympathetic interest in her later legal troubles: There is, of course, nothing indecent in that pamphlet; on the contrary, it is notably prudent and clean.

The author wrote it for the instruction of her own young sons, and its superiority to most other such literature was so apparent that it was reprinted at length in a medical journal, and circulated in great numbers by clergymen, Y.M.C.A.

Then Mrs. Dennett, who is engaged in birth-control propaganda, began annoying the wowsers of the U.S. Post Office by exposing their gross stupidity and disingenuousness in the enforcement of the Comstock Act, and they retorted by barring her pamphlet from the mails.

[43] Eventually a safe jury was empanelled by the prosecution “and Mrs. Dennett was quickly convicted, and Judge Burrows fined her $300.

[1] The American Civil Liberties Union (ACLU) supported and sponsored Dennett (its general counsel, Morris Ernst, was her defense attorney), maintaining that her pamphlet was not obscene.

Swan, Augustus Noble Hand and Chase, JJ, set aside the verdict, decided that the pamphlet was so obviously not obscene that ‘no case was made for submission to the jury,’ and ordered Mrs. Dennett released from her bond.” [45] When the United States Court of Appeals for the Second Circuit overturned her conviction in 1930, the Court set a legal precedent that took intent into account in the evaluation of obscenity.

She wishes for everyone to “concentrate all our energies on the real point- namely, that the qualifications for voting shall be made without regard to sex.”[46] Dennett wrote the pamphlet, “The Sex Side of Life'' in 1915.

Her main focus was to share her opinions and encourage a discussion concerning how necessary the laws pertaining to birth control are in the United States.

Frederick H. Hitchcock wrote in 1936, “This book is obviously propaganda for the removal of federal and state restrictions on the importation, manufacture and distribution of devices and materials wherewith to prevent conception and for the removal of restrictions on the dissemination of information concerning the prevention of conception.” [51] James E. Wilkinson was the prosecutor during Dennett’s trial for sending her pamphlet through the US mail.

Prosecuting attorney James Wilkinson described the booklet as "pure and simple smut" and stated "If I can stand between this woman and the children of the land I will have accomplished something".