On January 6, 1845, at the age of 18, she married Henry H. Gage, a merchant of Cicero, making their permanent home at Fayetteville, New York.

Gage became involved in the women’s rights movement in 1852 when she decided to speak at the National Women’s Rights Convention in Syracuse, New York. She served as president of the National Woman Suffrage Association from 1875 to 1876 and served as either Chair of the Executive Committee or Vice President for over twenty years. During the 1876 convention, she successfully argued against a group of police who claimed the association was holding an illegal assembly. They left without pressing charges.

Like many other suffragists, Gage considered abortion a regrettable tragedy, although her views on the subject were more complex than simple opposition. In 1868, she wrote a letter to The Revolution (a women’s rights paper edited by Elizabeth Cady Stanton and Parker Pillsbury), supporting the view that abortion was an institution supported, dominated and furthered by men. Gage opposed abortion on principle, blaming it on the ‘selfish desire’ of husbands to maintain their wealth by reducing their offspring:

Gage described herself as “born with a hatred of oppression.” As a result of the campaigning of the New York State Woman Suffrage Association under Gage, the state of New York granted female suffrage for electing members of the school boards. Gage ensured that every woman in her area (Fayetteville, New York) had the opportunity to vote by writing letters making them aware of their rights, and sitting at the polls making sure nobody was turned away. In 1871, Gage was part of a group of 10 women who attempted to vote. Reportedly, she stood by and argued with the polling officials on behalf of each individual woman. She supported Victoria Woodhull and (later) Ulysses S Grant in the 1872 presidential election. In 1873 she defended Susan B. Anthony when Anthony was placed on trial for having voted in that election, making compelling legal and moral arguments. In 1884, Gage was an Elector-at-Large for Belva Lockwood and the Equal Rights Party.

Gage was well-educated and a prolific writer—the most gifted and educated woman of her age, claimed her devoted son-in-law, L. Frank Baum. She corresponded with numerous newspapers, reporting on developments in the woman suffrage movement. In 1878, she bought the Ballot Box, the monthly journal of a Toledo, Ohio, suffrage association, when its editor, Sarah R. L. Williams, decided to retire. Gage turned it into The National Citizen and Ballot Box, explaining her intentions for the paper thus:

Gage was quite concerned with the rights of a woman over her own life and body. In 1881 she wrote, on the subject of divorce:

Gage unsuccessfully tried to prevent the conservative takeover of the women’s suffrage movement. Susan B. Anthony, who had helped to found the National Woman Suffrage Association (NWSA), was primarily concerned with gaining the vote, an outlook which Gage found too narrow. Conservative suffragists were drawn into the suffrage movement believing women’s vote would achieve temperance and Christian political goals. These women were not in support of general social reform. The American Woman Suffrage Association (AWSA), part of the conservative wing of the suffrage movement (and formerly at odds with the National), was open to the prospect of merging with the NWSA under Anthony, while Anthony was working toward unifying the suffrage movement under the single goal of gaining the vote. The merger of the two organizations, pushed through by Lucy Stone, Alice Stone Blackwell and Anthony, produced the National American Woman Suffrage Association (NAWSA) in 1890. Stanton and Gage maintained their radical positions and opposed the merger of the two suffrage associations because they believed it was a threat to separation of church and state. The successful merger of the two suffrage groups prompted Gage to establish the Woman’s National Liberal Union (WNLU) in 1890, of which she was president until her death (by stroke) in 1898. Attracting more radical members than NAWSA, the WNLU became the platform for radical and liberal ideas of the time. Gage became the editor of the official journal of the WNLU, The Liberal Thinker.

In 1893, she published Woman, Church and State, a book that outlined the variety of ways in which Christianity had oppressed women and reinforced patriarchal systems. It was wide-ranging and built extensively upon arguments and ideas she had previously put forth in speeches (and in a chapter of History of Woman Suffrage which bore the same name). Gage became a Theosophist, and the last two years of her life, her thoughts were concentrated upon metaphysical subjects, and the phenomena and philosophy of Spiritualism and Theosophical studies. During her critical illness in 1896, she experienced some illuminations that intensified her interest in psychical research. She had great interest in the occult mysteries of Theosophy and other Eastern speculations as to reincarnation and the illimitable creative power of man.

In her 1893 work, Woman, Church and State, she cited the Iroquois society, among others, as a ‘Matriarchate’ in which women had true power, noting that a system of descent through the female line and female property rights led to a more equal relationship between men and women. Gage spent time among the Iroquois and received the name Karonienhawi – “she who holds the sky” – upon her initiation into the Wolf Clan. She was admitted into the Iroquois Council of Matrons.

Maud, who was ten years younger than Julia, initially horrified her mother when she chose to marry author L. Frank Baum (The Wonderful Wizard of Oz) at a time when he was a struggling actor with only a handful of plays (of which only The Maid of Arran survives) to his writing credit. However, a few minutes after the initial announcement, Gage started laughing, apparently realizing that her emphasis on all individuals making up their own minds was not lost on her headstrong daughter, who gave up a chance at a law career when the opportunity for women was rare. Gage spent six months of every year with Maud and Frank. Gage’s son Thomas Clarkson Gage and his wife Sophia had a daughter named Dorothy Louise Gage, who was born in Bloomington, Illinois, on June 11, 1898, but died five months later, on November 11, 1898.

Gage died in the Baum home in Chicago, in 1898. Although Gage was cremated, there is a memorial stone at Fayetteville Cemetery that bears her slogan “There is a word sweeter than Mother, Home or Heaven. That word is Liberty.”

In 1993, scientific historian Margaret W. Rossiter coined the term “Matilda effect”, after Matilda Gage, to identify the social situation where woman scientists inaccurately receive less credit for their scientific work than an objective examination of their actual effort would reveal. The “Matilda effect” is the opposite of the “Matthew effect”, in which scientists already famous are over-credited with new discoveries. Gage’s legacy was detailed in biographies published by Sally Roesch Wagner, and Charlotte M. Shapiro.

In 1995, Gage was inducted into the National Women’s Hall of Fame.

The death so upset the child’s aunt Maud, who had always longed for a daughter, that she required medical attention. Thomas Clarkson Gage’s child was the namesake of her uncle Frank Baum’s famed fictional character, Dorothy Gale. In 1996, Dr. Sally Roesch Wagner, a biographer of Matilda Joslyn Gage, located young Dorothy’s grave in Bloomington. A memorial was erected in the child’s memory at her gravesite on May 21, 1997. This child is often mistaken for her cousin of the same name, Dorothy Louise Gage (1883–1889), Helen Leslie (Gage) Gage’s child.