Chrystal Macmillan, the only daughter in the family of nine children of John Macmillan, a wealthy tea merchant, was born in Edinburgh on 13th June 1882. She was educated at St Leonards School, and in 1892 was among the first women admitted to University of Edinburgh, where in 1896 she took a BSc with first-class honours in mathematics and natural philosophy.
In 1902 she was a member of the National Union of Women's Suffrage Societies (NUWSS), the Edinburgh Ladies Debating Society and honorary secretary and treasurer of the Committee of Women Graduates of the Scottish Universities. A fellow member was Elsie Inglis.
After reading The Enfranchisement of Women, she contacted the author of the pamphlet, Elizabeth Wolstenholme-Elmy, for advice on women's suffrage. Under legislation of 1868 the Scottish universities had four MPs; the electorate was the general councils of the universities, which included all their graduates. Macmillan therefore decided to test out that the word "person", used throughout the statute, included women. In 1908 she became the first woman to plead before the House of Lords when she advocated that women graduates should be given the vote. According to her biographer, Sybil Oldfield: "Not herself a lawyer at this time, she showed considerable skill in presenting her case, but the case was rejected."
As a result of the case Chrystal Macmillan published the pamphlet, The Struggle for Political Liberty (1909). She moved to London, where she served on the executive of the National Union of Women's Suffrage Societies. Although she was a suffragist she did send a donation to the Women Social & Political Union in 1909. She was also an active member of the Women's Freedom League. In 1913 she became vice-president to the International Women's Suffrage Alliance.
Over the next few years she appeared before several committees of enquiry on matters of concern to women including those on street offences and unemployment insurance. Macmillan was also involved in the campaign against the law that resulted in women losing their citizenship when they married a foreigner.
In July 1914 the NUWSS argued that Asquith's government should do everything possible to avoid a European war. Two days after the British government declared war on Germany on 4th August 1914, Millicent Fawcett declared that it was suspending all political activity until the conflict was over. Although the NUWSS supported the war effort, it did not follow the WSPU strategy of becoming involved in persuading young men to join the armed forces. Despite pressure from members of the NUWSS, Fawcett refused to argue against the First World War. Her biographer, Ray Strachey, argued: "She stood like a rock in their path, opposing herself with all the great weight of her personal popularity and prestige to their use of the machinery and name of the union."
Sybil Oldfield has argued: "Chrystal Macmillan was a committed internationalist, believing that what all people had in common was more important than the frontiers that divided them. Immediately before the outbreak of war, she helped draft an international manifesto of women, signed by the representatives of twelve million women, and delivered it to the foreign secretary, Sir Edward Grey, and the European ambassadors in London on 31 July 1914."
At a Council meeting of the National Union of Women's Suffrage Societies held in February 1915, Millicent Fawcett attacked the peace efforts of people like Mary Sheepshanks. Fawcett argued that until the German armies had been driven out of France and Belgium: "I believe it is akin to treason to talk of peace." After a stormy executive meeting in Buxton all the officers of the NUWSS (except the Treasurer) and ten members of the National Executive resigned over the decision not to support the Women's Peace Congress at the Hague. This included Chrystal Macmillan, Kathleen Courtney, Margaret Ashton, Catherine Marshall, Eleanor Rathbone and Maude Royden, the editor of the The Common Cause.
In April 1915, Aletta Jacobs, a suffragist in Holland, invited suffrage members all over the world to an International Congress of Women in the Hague. Some of the women who attended included Chrystal Macmillan, Mary Sheepshanks, Jane Addams, Alice Hamilton, Grace Abbott, Emily Bach, Lida Gustava Heymann, Emmeline Pethick-Lawrence, Kathleen Courtney, Emily Hobhouse, Rosika Schwimmer. At the conference the women formed the Women's International League for Peace and Freedom (WIL).
After the Armistice Macmillan was a delegate at the Paris Peace Conference. In May 1919 she was a delegate to the International Congress of Women in Zürich, which issued the first public criticism of the punitive terms of the Versailles Treaty.
In 1924 Macmillan was called to the English Bar, where she practised as a barrister, specializing in the nationally laws of married women. Cicely Hamilton argued: "She was the right kind of lawyer, one who held that Law should be synonymous with Justice … Her chief aim in life - one might call it her passion - was to give every woman of every class and nation the essential protection of justice. She was, herself, a great and very just human being… She could not budge an inch on matters of principle but she never lost her temper and never bore a grudge in defeat."
As Elizabeth Crawford, the author of The Suffragette Movement (1999) has pointed out: "In 1923 she was a founder member of the Open Door Council, working for equal employment opportunities of women, and in 1929 was a founder member and president of Open Door International." Macmillan unsuccessfully stood as a Liberal for Edinburgh North in the 1935 General Election.
In June 1937 Chrystal Macmillan had to have a leg amputated. She died following a heart-attack at her home at 8 Chalmers Crescent, Edinburgh, on 21st September 1937.