At the age of eighteen, Marie Stopes won a science scholarship at University College, London. Marie was a talented and committed student and in 1901 achieved a double first in botany. She proudly wrote to her mother: "I have got my degree. I am now B.Sc. Not only have I got it, I have got it well… I got Honours in Botany and Geology… I am the only candidate with honours; the others (men only) all failed, so my name stands alone in the list. It is supposed to be impossible to take one honours in a year, to get two is nice." She continued her studies and in 1905 she obtained her DSc and became Britain's youngest doctor of Science.

Although very involved in her academic work, Marie Stopes was also interested in politics. Like her mother she supported the women's suffrage campaign and eventually joined the Women's Freedom League. This brought her into conflict with her mother, who was a member of the more militant Women's Social and Political Union. Marie believed in the non-violent approach and was never sent to prison for her beliefs.

After several unsuccessful love affairs, Marie married the Canadian botanist, Reginald Gates on 18th March, 1911. However, she took the unusual step at the time of retaining her surname. Unlike Marie, Gates held traditional views of how women should behave. He strongly opposed her support for women's suffrage. The marriage was annuled for non-consummation in 1916.

During the First World War Marie began writing a book about feminism and marriage. In her book Married Love, Marie argued that marriage should be an equal relationship between husband and wife. However, she had great difficulty finding a publisher. Walter Blackie of Blackie & Son rejected her manuscript with the words: "The theme does not please me. I think there is far too much talking and writing about these things already… Don't you think you should wait publication until after the war? There will be few enough men for the girls to marry; and a book like this would frighten off the few." Blackie objected to passages such as, "far too often, marriage puts an end to women's intellectual life. Marriage can never reach its full stature until women possess as much intellectual freedom and freedom of opportunity within it as do their partners."

It was not until, March 1918, that Marie Stopes found a small company that was willing to take the risk of publishing Married Love. It later emerged that the money was put up by a businessman, Humphrey Verdon Roe (1878–1949). The book was an immediate success, selling 2,000 copies within a fortnight and by the end of the year had been reprinted six times. The book was also published in America but the courts declared the book was obscene and it was promptly banned.

Marie's next book was about birth-control. She had become interested in this subject after meeting Margaret Sanger, a birth-control campaigner from America. Sanger had been converted to socialism, while working as a nurse in the slums of New York City. She observed that many women died of self-induced abortions or raised large families in poverty. Sanger began publishing her own newspaper where she argued in favour of birth-control and abortion. The main theme of her articles was that "no woman can call herself free who doesn't own and control her own body." After advice about birth-control appeared in her newspaper in 1915, she was charged with publishing an "obscene and lewd article". Margaret Sanger fled to Britain and it was while she was in London she met Marie Stopes.

In 1918 Stopes wrote a concise guide to contraception called Wise Parenthood. Marie Stopes' book upset the leaders of the Church of England who believed it was wrong to advocate the use of birth control. Roman Catholics were especially angry, as the Pope had made it clear that he condemned all forms of contraception. Despite this opposition, Marie continued her campaign and in 1921 founded the Society for Constructive Birth Control and Racial Progress. With financial help from her rich second husband, Humphrey Roe, Marie also opened the first of her birth-control clinics in Holloway on 17th March 1921. She knew it would be dangerous as several people in Britain, including Richard Carlile, Charles Bradlaugh and Annie Besant, had been sent to prison for advocating birth-control.

Although Marie Stopes was not prosecuted, Guy Aldred and Rose Witcop, who published a pamphlet written by Margaret Sanger, were found guilty of selling an obscene publication. The case drew much press coverage and the couple were supported financially by John Maynard Keynes, Dora Black and Bertrand Russell. Later that year this group was joined by Katharine Glasier, Susan Lawrence, Margaret Bonfield, Dorothy Jewson and H. G. Wells to establish the Workers' Birth Control Group.

Many Roman Catholics believed that Marie should also be charged with an offence. Halliday Sutherland wrote in an article in The Daily Express where he called for her to be sent to prison. Sutherland also wrote a book, Birth Control a Statement of Christian Doctrine Against the Neo Malthusians, where he accused Marie Stopes of writing obscene books. Stopes sued him for libel and although she lost the initial case, she appealed and won. Later the decision was overturned by the House of Lords.

Marie Stopes became a target for Roman Catholic abuse. One priest complained: "Madam… I consider it most useful to pray God that your writings may not do as much injury to morals - to the ignorant poor especially - which they are calculated to do… It appears to me that a pagan might have written as you do… I had hopes no women would write such books." The Catholic Truth Society argued: "Our faithful Catholic mothers are doing wonderful work for God. In time, if methods of birth control continue to prevail among the non-Catholics, their race will die out and the Catholic race will prevail and thus England will become again what it once was, a Catholic country."

Marie Stopes was involved in several other crusades during her life. This included an attempt to stop education authorities from sacking married women teachers. Marie also become involved in the campaign to persuade the Inland Revenue to tax husbands and wives separately. Lesley A. Hall has argued: "Although Stopes was fast approaching forty, she did not moderate her hectic round of lecturing, writing (including a short pamphlet, A Letter to Working Mothers (1919), explaining birth control in simple language and terms suited to the circumstances of poor women), travelling, dealing with her enormous correspondence, dashing off letters to the newspapers, and joining the deliberations of the National Birth Rate Commission. She still pursued her researches on the composition of coal, undertaken as her war work, at the Home Office experimental station at Eskmeals, Cumberland, at University College, and in a makeshift home laboratory. She published several important papers on the subject and her nomenclature for the four constituents of coal (vitrain, clarain, durain, and fusain) became standard usage."

However, he most controversial campaign concerned eugenics, then not a discredited science. Stopes joined the Eugenics Education Society in 1912 and In Radiant Motherhood she argued that the "sterilisation of those totally unfit for parenthood to be made an immediate possibility, indeed made compulsory." In 1935 Stopes visited Nazi Germany and attended the International Congress for Population Science in Berlin. Stopes was now attacked by some of her former supporters such as Guy Aldred and Havelock Ellis.

Her biographer, Lesley A. Hall has argued: "Of recent years Stopes has been much decried for eugenicist views. A member of the Eugenics Society for many years (she left it her clinics lest they should fall into the hands of the Family Planning Association), like many of her contemporaries she held strong views about good and bad breeding. However, she argued that the majority of women, given the chance to space their pregnancies, would bear healthier children and rear them better. Moreover, the effect of her eugenicist views on the practice within her clinics, and on the advice she gave to correspondents, seems minimal: her sympathy towards individual cases often contradicted her public pronouncements."

Stopes spent the rest of her life campaigning for the causes she believed in. Much of her time was spent writing articles for her newspaper Birth Control News. Marie also wrote novels and poetry. This included Love's Creation (1928) and Love Songs for Young Lovers (1938).

Marie Stopes died of breast cancer on 2nd October, 1958.