Inez was born March 2, 1873, in Rio de Janeiro, where her parents had relocated from New England in hopes of becoming financially successful in the coffee business. Their efforts failed when she was very young and they moved back to Boston.
She was the ninth of 12 children and from observing her mother's life of toil and childbearing she developed "a profound horror of the women's life" that formed the basis for her lifelong feminist views. At 14, while researching the topic "Should Women Vote?" for a school paper, she became a confirmed suffragist.
She married Rufus Hamilton Gillmore in 1897, and at the same time, entered Radcliffe College founding the College Equal Suffrage League which organized undergraduates for the cause of suffrage. Upon graduation, she and her husband moved to New York City becoming leaders in the avant-garde Greenwich Village community. Inez published magazine articles and short stories and in 1908 her first novel, June Jeopardy.
During this time, she met William Henry Irwin managing editor of McClure's Magazine. She left her first husband in 1913, obtained a divorce and married Irwin in 1916.
She now turned to writing full time and accompanied her husband to Europe during World War I, reporting on the progress of the war in Italy and France for American magazines.
In 1921, she published The Story of the Woman's Party, an inspiring history of the suffrage campaign, produced 12 novels and in 1924 won the O.Henry Memorial Prize for her short story "The Spring Flight". Her biggest success was her Maida series of children's books in 1910 (Maida's Little Shop) and ended with the 11th volume in 1951.