Mary Agnes Adamson, commonly known as 'Molly', was born in Withington, Manchester, on 8 July 1882, the eldest of six children of Robert Adamson, philosopher, and his wife Margaret, née Duncan, a botanist and secondary school teacher. The family moved back to Scotland in 1889 and Molly was educated at Aberdeen and Glasgow Girls' High Schools before attending the University of Kiel in 1901 for seven months to learn German. She went up to Newnham College, Cambridge (where her mother had also been a student) in 1901 to read Classics, then Economics as part of the History tripos, for which she received first-class honours. In January 1905 she became an assistant in the History department at University College of South Wales, Cardiff and in September of that year married an economist colleague, Charles Joseph Hamilton, later a barrister and journalist. She petitioned for and obtained a divorce in March 1914, retaining her married name.

Mary Agnes Hamilton's early political career was shaped by her involvement in suffragist and socialist circles, her public debating skills having been honed in discussions with her four sisters and a society at Newnham called 'Things That Matter'. Initially an ardent pacifist, she was an original member of the Union of Democratic Control. She joined the Independent Labour Party in 1914, helping to draft its constitution, and then became an individual member of the Labour Party in 1918, where she went on to work most closely with the trade unionists. She stood unsuccessfully as Labour candidate for Chatham in 1923 and Blackburn in 1924. Between 1924 and 1929, she was a member of the Balfour Committee on Industry and Trade.

In the 1929 general election Hamilton won one of two seats for Blackburn, securing the highest number of votes of any woman candidate. During her time as a Labour MP, she served on the Royal Commission on the Civil Service, 1929-31, and was parliamentary private secretary to the Postmaster General, Clement Attlee. She was also appointed a delegate to the League of Nations in Geneva, where in 1929 and 1930 she worked on the Refugees Commission and the International Committee on Intellectual Cooperation. She did not join the National Government in August 1931, and was instead elected to the Labour Party's parliamentary executive, briefly serving as front-bench spokesman on budget issues for the Opposition. She lost her seat in the general election of October 1931. She was later elected as a Labour alderman to the London County Council, 1937-1939.

Throughout her life, Hamilton supported herself through her writing and journalism. She joined the staff of the Economist in 1913, moving to the journal Common Sense with her editor F. W. Hirst in 1916, and later became an assistant editor at the Review of Reviews, 1920-22, and the I.L.P.'s journal New Leader, 1922. Her early book-length publications included translations of historical works in French and German, and history books for children. She wrote ten novels, some of which drew on her experience in politics. She also published a series of political biographies of notable individuals in the history of socialism, and later went on to write two autobiographical works which combined life-writing with contemporary history.

During the 1930s, Hamilton extended her work across a range of activities including broadcasting and lecturing. Most notably, she undertook a number of commercial lecture tours in the United States, where she consulted with individuals involved in progressive politics and social work, including American student friends from Newnham. She had presented the first 'Week in Westminster' for the BBC in 1929 and she continued to broadcast talks on the BBC on current affairs, international relations, and professional careers for women, among many other topics. She was made a governor of the BBC, 1933-37, and was a member of the Brains Trust. She was also appointed to the expert advisory board of the Cambridge University women’s careers service, 1930-40, and worked closely with Ray Strachey in establishing the Women's Employment Federation.

During and after the Second World War, Hamilton was employed as a temporary civil servant. In February 1940, she joined the Production Department of the Ministry of Information, where she was later transferred to the General Division, at the instigation of Kenneth Clark, to work on literary propaganda and morale. In February 1941, she became a member of the Reconstruction Secretariat, later the Ministry of Reconstruction, where she continued to write public information pamphlets and served on planning committees for Education, Employment for the Disabled, and the Beveridge Report (Committee on Social Insurance and Allied Services). Between November 1942 and March 1943, she was sent, as head of British Information Services, on a speaking tour of the United States to promote the British war effort. She also organised William Beveridge's Rockefeller Foundation tour of the United States to promote the Beveridge report. In May 1944, Hamilton moved back to the American Division of the Ministry of Information, where she served on the Films Advisory Committee. In August 1946, the American Division, of which she was then in charge, was transferred to the Foreign Office. In 1947, she gave the fifth Montague Burton Lecture on International Relations, on 'The Place of the United States of America in World Affairs'. She left the civil service in February 1952, retiring from the public eye in later life due to ill health.

Mary Agnes Hamilton was made a C.B.E. for her work as a civil servant in 1949.

She died at home in Ealing, London, on 10 February 1966.

Her non-fiction publications include: The Story of Abraham Lincoln (1906); A Junior History of Rome (1910); Greek Legends (1912); Outlines of Greek and Roman History to A.D. 180 (1913); The Investment of Capital Abroad (1915); The Principles of Socialism (1921); Follow My Leader (1922); Ancient Rome (1922); The Man of To-morrow: J. Ramsay MacDonald, under pseudonym (1923); Margaret Bondfield, under pseudonym (1924); Mary Macarthur (1925); Thomas Carlyle (1926); In America To-Day (1932); Sidney and Beatrice Webb (1933); John Stuart Mill (1933); (ed.) The Boat Train: By Fifteen Travellers (1935); 'Changes in Social Life' in Our Freedom and Its Results by Five Women, ed. Ray Strachey (1936); Newnham: An Informal Biography (1936); Arthur Henderson (1938); The Labour Party To-Day (1939); Women at Work: A Brief Introduction to Trade Unionism for Women (1941); Remembering My Good Friends (1944); and Up-Hill All the Way: A Third Cheer for Democracy (1953).

Her novels include: Less than the Dust (1912); Yes (1914); Dead Yesterday (1916); Full Circle (1918); The Last Fortnight (1920); Follow My Leader (1922); Folly's Handbook (1927); Murder in the House of Commons (1931); Special Providence: A Tale of 1917 (1930); Life Sentence (1935)