Born in Chatsworth, Ontario in 1873, she later moved with her family to a homestead in the Souris Valley of Manitoba. Between 1904 and 1911, Nellie McClung, her husband Wesley (a druggist) and their five children resided in Manitou, Manitoba. The women’s rights movement in Winnipeg embraced her. An effective speaker with a sense of humour, she played a leading role in the successful Liberal campaign in 1914. She lived in the West for the rest of her life in Manitou, Winnipeg, Edmonton, Calgary and Victoria. McClung was the grandmother of outspoken Alberta judge John McClung. The Manitou house in which McClung and her family lived has been re-located to the Archibald Historical Museum in La Riviere, Manitoba where it has been restored. The house is open to the public.

McClung once said "Why are pencils equipped with erasers if not to correct mistakes?", when arguing for the support of equitable divorce laws, of which she was a long time supporter.

Her great causes were women's suffrage and the temperance. She understood that the First World War had played an important role in broadening the appeal of women's suffrage because the manpower shortages required widespread female employment, making the image of the sheltered female more obviously inapplicable to Canadian circumstances. It was largely through her efforts that in 1916 Manitoba became the first province to give women the right to vote and to run for public office. The Government of Canada followed suit that same year. After moving to Edmonton, Alberta, she continued the campaign for suffrage. She championed dental and medical care for school children, property rights for married women, mothers' allowances, factory safety legislation and many other reforms. McClung was a supporter of the then popular social philosophy of eugenics and campaigned for the sterilization of those considered "simple-minded". Her promotion of the benefits of sterilization contributed to the passage of eugenics legislation in Alberta.

She published her first novel Sowing Seeds in Danny in 1908. A national bestseller, it was succeeded by short stories and articles in several Canadian and American magazines. She served as a Liberal member of the Legislative Assembly of Alberta from 1921 to 1926. As an opposition member, her opportunity to press for women's rights was limited, because women were not taken seriouslyShe was one of The Famous Five (also called The Valiant Five), with Irene Parlby, Henrietta Muir Edwards, Emily Murphy and Louise McKinney. The five put forward a petition, in 1927, to clarify the term "Persons" in Section 24 of the British North America Act 1867. This section had served to exclude women from political office. The petition was successful, clearing the way for women to enter politics in Canada.

Among other honours, in October 2009, the Senate voted to name McClung and the rest of the Five Canada's first "honorary senators."

Fiction

• Sowing Seeds in Danny (1908)
• The Second Chance (1910)
• The Black Creek Stopping House and Other Stories (1912)
• Purple Springs (1921)
• When Christmas Crossed 'The Peace' (1923)
• Painted Fires (1925)
• All We Like Sheep (1926)
• Be Good to Yourself (1930)
• Flowers for the Living (1931)

Non-Fiction

• In Times Like These (1915)
• The Next of Kin (1917)
• Three Times and Out (1918)
• Clearing in the West (1935)
• Leaves from Lantern Lane (1936)
• More Leaves from Lantern Lane (1937)
• The Stream Runs Fast (1945)
• The Morning After Dawn (1950)
• AFG FOR LIFE